Re: A homemade diode power sensor for HP meters
Dr. David Kirkby from Kirkby Microwave Ltd
On Sat, 17 Nov 2018, 17:30 Peter Gottlieb < hpnpilot@... wrote: This is an interesting project.? As you say the meters themselves are dirt
cheap.? I have seen blown sensors sold cheap as well and maybe it's possible to
build your circuit into one of those which would have the advantage of
connectors and case already taken care of.? If a small inexpensive SM board is
made to do this it could easily fit and be so inexpensive that it wouldn't even
be worth the time to troubleshoot a failure, just replace the board if it gets
blown out.
Peter
The worrying thing is the Chinese counterfeiters will probably do just that. eBay seller yixunhk springs to mind - someone called them the HP rebirthing centre. I can see them "rebirthing" power sensors. ?
Dave
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Re: A homemade diode power sensor for HP meters
Hi Tobias, I looked at the OP again, and realized that you already knew about the "mount" resistor - I didn't notice that the first time. I grabbed that 8484A manual pdf that Orin linked - thanks Orin, I have lots of 8484As, and the manual somewhere, but can never seem to find it. Now I have a fresh copy.
Anyway, after refreshing my memory on these sensors, and looking also at the 8481A schematic, I'd recommend studying both types - the TC and diode models have a different arrangement for the auto-zeroing function. The circuit you posted puts the AZ feedback to the detector, as in the 8481A TC circuit, but that won't work right. With the TC type, it is a low impedance source, so the AZ current can go through, and offset the signal. With a diode detector, you should use a circuit like in the 8484A, which applies it to the ground-side JFET switch in the chopper. The diode detector is a high impedance source, so you don't want to put the AZ feedback current there.
Good luck,
Ed
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Readings from 436A power meter using 82357B USB-GPIB?
Hello,
Just got hold of an old HP436A power meter and would like to do some automated measurements. Controlling the instrument is simple using the 82357B USB-GPIB module (I can set range etc), but reading data? As a first step I'd like to test using Agilent Interactive IO from Connection Expert. If that works, I'm fairly sure I can get it working in a program also.?
The 436 is set to NORMAL, i.e., not TALK ONLY. It does have the HP-IB option 022 installed. My guess is that I need to address the instrument to Talk in some other way than the "Read Response" used in Interactive IO.
Anyone with experience of this?
Regards,
? Staffan
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Re: A homemade diode power sensor for HP meters
Hi Robert,
yes, I have an extra diode. While the RF has negative polarity, the 3.3nF capacitor gets charged through the bottom diode. As soon as the RF has positive polarity, the top diode conducts, and the voltage across the 3.3nF capacitor adds to the RF voltage. So this detector works like a kind of charge pump, offering a higher output voltage at the same RF level. I hope to improve the sensitivity with this.
Concerning the terminator - yes I forgot to add a 50 Ohm resistor in my schematic. And, besides that, I used an external 6dB pad at the input, which I probably should have mentioned earlier. This improves return loss and definitely provides a DC path for the diodes.
Tobias
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Re: A homemade diode power sensor for HP meters
Hi ,
it seems to me that q1 and q2 (FET) form an electronic key. then the capacitors must be there for the switch to work properly without switching aberrations. Already C7 and C8, possibly keep the input impedance controlled and prevent q1 (bjt) from oscillating.
Em sábado, 17 de novembro de 2018 17:49:59 BRST, Robert G8RPI via Groups.Io <robert8rpi@...> escreveu:
You seem to have an extra diode, no terminator and reversed polarity. The terminator is probably the issue.
The 11863A calibrator manuals may give some clues too.
Robert G8RPI.
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Re: A homemade diode power sensor for HP meters
You seem to have an extra diode, no terminator and reversed polarity. The terminator is probably the issue.
The 11863A calibrator manuals may give some clues too.
Robert G8RPI.
|
Re: A homemade diode power sensor for HP meters
Orin beat me to it but BAMA also have a copy (with some the
change sheets) at:
I bought a box of a dozen dead 84xx sensors a while back for
about 60 USD, with much the same idea in mind but haven't got
round to it yet! I would happy to ship one over to you if it would
help?
Adrian
On 11/17/2018 7:19 PM, Tobias Pluess
wrote:
toggle quoted message
Show quoted text
Robert,
do you own a schematic of the diode sensors? The schematic
for the thermocouple sensors was hard enough to obtain. In the
current service manuals, there are of course no schematics at
all, and the HP journals of that time also don't show the actual
schematics of the sensors.
Tobias
Sent from
my Samsung Galaxy smartphone.
-------- Original message --------
Date: 11/17/18 20:09 (GMT+01:00)
Subject: Re: [HP-Agilent-Keysight-equipment] A homemade diode
power sensor for HP meters
Interesting project. The diagram you show is for the
Thermocouple "A" suffix sensor. While the principle and basic
circuit is the same for diode and thermocouple, the A2 PCB is
different so there must be a difference in the circuit. This may
be related to your offset issue.
Robert G8RPI.
|
Re: A homemade diode power sensor for HP meters
8484A diode sensor schematic is here...
Robert,
do you own a schematic of the diode sensors? The schematic for the thermocouple sensors was hard enough to obtain. In the current service manuals, there are of course no schematics at all, and the HP journals of that time also don't show the actual schematics
of the sensors.
Tobias
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Re: A homemade diode power sensor for HP meters
Robert,
do you own a schematic of the diode sensors? The schematic for the thermocouple sensors was hard enough to obtain. In the current service manuals, there are of course no schematics at all, and the HP journals of that time also don't show the actual schematics
of the sensors.
Tobias
Sent from my Samsung Galaxy smartphone.
toggle quoted message
Show quoted text
-------- Original message --------
From: "Robert G8RPI via Groups.Io" <robert8rpi@...>
Date: 11/17/18 20:09 (GMT+01:00)
To: HP-Agilent-Keysight-equipment@groups.io
Subject: Re: [HP-Agilent-Keysight-equipment] A homemade diode power sensor for HP meters
Interesting project. The diagram you show is for the Thermocouple "A" suffix sensor. While the principle and basic circuit is the same for diode and thermocouple, the A2 PCB is different so there must be a difference in the circuit. This may be related
to your offset issue.
Robert G8RPI.
|
Re: A homemade diode power sensor for HP meters
Interesting project. The diagram you show is for the Thermocouple "A" suffix sensor. While the principle and basic circuit is the same for diode and thermocouple, the A2 PCB is different so there must be a difference in the circuit. This may be related to your offset issue.
Robert G8RPI.
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Re: Yet another TG project
I have added a new picture, showing the low frequency end performance:
/g/HP-Agilent-Keysight-equipment/photo/78668/1?p=Name,,,20,1,0,0
The upper trace A is the TG output near 0 dBm, up to 1 kHz. The lower B trace is the stored maximum hold, with the TG output "OFF." The line spurs are artifacts due to ground loops in the experimental test and repair setups - I'm still working on the sweep-stopping issue, etc. In earlier tests before this came up, the TG exhibited no line/PS stuff at all - only the residual line noise of the 8568 was evident, about 10 dB lower than it looks here. It should be OK once everything is buttoned up properly.
The TG/SA arrangement exhibits nearly 100 dB dynamic range, and is at the limit of on-screen capability. The near-DC (f=0) response of the 8568 at the narrowest IFBW hits the noise floor by about 100 Hz, so it's fully usable from there up. The TG output is of course, coming from the low-band (~0-500 MHz) amplifier. The original, main amplifier rolls off below around 300 kHz, so is incapable of reaching the bottom end of the 8568's frequency range.
Ed
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Re: A homemade diode power sensor for HP meters
@Peter
thanks. Yes, exactly, the meters themselves are really cheap. I currently don't have access to some blown 8481A or similar sensors, so I cannibalized a sensor cable :-( because the connectors seem to be HP specific ones and are not available e.g. at Mouser or so.
But yes, as soon as I have a working circuit, I will it design such that it fits into the normal housing for those power sensors. I also thought of milling my own housing which would be similar to the original HP housing anyways.
The cool stuff is that Agilent and Keysight still use the same interface (at least for some power meters) as far as I know, so the sensor should work with any of those!
@Ed
?thanks also. Yes, I tried to study on the interface between the 436 and the power sensor. The resistor you are talking about is called the mount resistor in the 8481A manual. Here is the general schematic of this sensor:
The service manual tells which resistor value is used for the different ranges. I think it will be possible on the homemade sensor to add a little switch to change the mount resistor value and therefore the range the meter uses - such, the same sensor could be used for different power ranges. The 436A power ranges are very limited, as far as I remember.
However, what I don't understand in the interface between the meter and the sensor is how the autozero circuit works, and I am also a bit unsure about the working principle of the amplifier formed by Q1 in the sensor (see schematic) and the OpAmp in the meter. Do you know more details about how it works?
Best
Tobias HB9FSX
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Re: A homemade diode power sensor for HP meters
Very cool, Tobias.
You may want to study up on the interface between the 436 and sensor heads. There's typically a particular resistor in each head type for identification by the 436 - it then figures out the scaling based on which head is installed. You should be able to fool it into thinking it's something else, close enough to work with your circuit.
Good luck,
Ed
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Re: A homemade diode power sensor for HP meters
This is an interesting project.? As you say the meters themselves are dirt cheap.? I have seen blown sensors sold cheap as well and maybe it's possible to build your circuit into one of those which would have the advantage of connectors and case already taken care of.? If a small inexpensive SM board is made to do this it could easily fit and be so inexpensive that it wouldn't even be worth the time to troubleshoot a failure, just replace the board if it gets blown out.
Peter
toggle quoted message
Show quoted text
On 11/17/2018 11:52 AM, Tobias Pluess wrote: Hi guys,
as you all know, the HP 435A, 436A, 437A and so on are quite nice power meters and the meter itself is availabe at quite low prices. However, the power sensors are quite expensive. I own a 8481A? and a 8484A, both in nice and working condition, but I don't want to damage anything so I treat them like raw eggs and handle them extremely careful. So I always thought whether it would be possible to make a own diode power sensor which is compatible to the HP 436A. This would have the advantage that the 436A could be used (and therefore its HPIB capabilities), and if it works with the 436A, it will do so also with the other meters (and possibly others, I don't know). Further, the homebrew power sensor would be cheap and if it breaks, no matter, it can be repaired easily.
So today I spent a few hours on making my own power sensor. I checked the schematic of the 8481A power sensor, and I did basically the very same. I came up with this schematic: (I don't want to waste valueable web space for the group, so I uploaded the high resolution pictures to my webspace)
The letters on the right side are the connector pins for the HP 436A, of course. Then I made a quick 'dead bug' style prototype, as follows.... (please don't laugh at me :-) ).
I connected to the 436A and I also installed a SMA 30dB attenuator at the input and connected my power sensor to the power reference of the 436A. At first, the readings were quite off a bit, but using the little trim pot on my PCB, I was able to precisely adjust the gain of my sensor such that the reading on the power meter was exactly 0.00 dBm. I then connected a signal generator to the power sensor and tested various power levels between -10dBm and +20dBm. Incredible, but my power sensor was accurate to 0.1dB! I also tested different frequencies between 1MHz and 2.6GHz. The flatness was not extremely good, but between +/-1dB. So, basically, the concept seems to work. I also tested different ranges for the power sensor - there is a 'mount resistor' inside the HP power sensors (8481, 8482, 9494, ...) which tells the power meter which range the sensor uses; the meter then internally adjusts the gain according to the power sensor range.
It appears that it should be possible to home brew such power sensors. However, at the moment, I have two issues:
a) the auto zero function does not work - even if I run the autozero feature, the power meter afterwards displays -13dBm anyway, so I cannot properly zero my power sensor. At the moment, I do not understand why this is so. Has anyone on the list an idea?
b) I don't understand exactly how the amplifier using the NPN transistor works. When I was adjusting the gain of my sensor, I tried to change the collector resistor (330 Ohms), but this had almost no effect on the gain. The emitter resistor (1k Ohms) also had only little effect on the gain. Only when I placed the pot where it is now I was able to adjust the gain in a wide range. So it basically looks to me like this amplifier is a common emitter amplifier, with the gain being RC / RE, is that true? why then is a feedback from the emitter required to the meter?
Thanks for any hints, tips and so on,
Tobias HB9FSX
|
A homemade diode power sensor for HP meters
Hi guys,
as you all know, the HP 435A, 436A, 437A and so on are quite nice power meters and the meter itself is availabe at quite low prices. However, the power sensors are quite expensive. I own a 8481A? and a 8484A, both in nice and working condition, but I don't want to damage anything so I treat them like raw eggs and handle them extremely careful. So I always thought whether it would be possible to make a own diode power sensor which is compatible to the HP 436A. This would have the advantage that the 436A could be used (and therefore its HPIB capabilities), and if it works with the 436A, it will do so also with the other meters (and possibly others, I don't know). Further, the homebrew power sensor would be cheap and if it breaks, no matter, it can be repaired easily.
So today I spent a few hours on making my own power sensor. I checked the schematic of the 8481A power sensor, and I did basically the very same. I came up with this schematic: (I don't want to waste valueable web space for the group, so I uploaded the high resolution pictures to my webspace)
The letters on the right side are the connector pins for the HP 436A, of course. Then I made a quick 'dead bug' style prototype, as follows.... (please don't laugh at me :-) ).
I connected to the 436A and I also installed a SMA 30dB attenuator at the input and connected my power sensor to the power reference of the 436A. At first, the readings were quite off a bit, but using the little trim pot on my PCB, I was able to precisely adjust the gain of my sensor such that the reading on the power meter was exactly 0.00 dBm. I then connected a signal generator to the power sensor and tested various power levels between -10dBm and +20dBm. Incredible, but my power sensor was accurate to 0.1dB! I also tested different frequencies between 1MHz and 2.6GHz. The flatness was not extremely good, but between +/-1dB. So, basically, the concept seems to work. I also tested different ranges for the power sensor - there is a 'mount resistor' inside the HP power sensors (8481, 8482, 9494, ...) which tells the power meter which range the sensor uses; the meter then internally adjusts the gain according to the power sensor range.
It appears that it should be possible to home brew such power sensors. However, at the moment, I have two issues:
a) the auto zero function does not work - even if I run the autozero feature, the power meter afterwards displays -13dBm anyway, so I cannot properly zero my power sensor. At the moment, I do not understand why this is so. Has anyone on the list an idea?
b) I don't understand exactly how the amplifier using the NPN transistor works. When I was adjusting the gain of my sensor, I tried to change the collector resistor (330 Ohms), but this had almost no effect on the gain. The emitter resistor (1k Ohms) also had only little effect on the gain. Only when I placed the pot where it is now I was able to adjust the gain in a wide range. So it basically looks to me like this amplifier is a common emitter amplifier, with the gain being RC / RE, is that true? why then is a feedback from the emitter required to the meter?
Thanks for any hints, tips and so on,
Tobias HB9FSX
|
Re: Yet another TG project
Thanks Don,
That is very interesting regarding the sweep digitizing process - I'll have to look into that.
Regarding the LO shifting process, I was vaguely aware of it, seeing it mentioned years ago, and apparently you can tell the 8568 not to do it for some situations like using the 8444A. Including the second LO info in the TG makes it so I don't have to care about that - it always gets the right answer.
The tracking stability of my TG/SA setup is looking very good - all that remains for instability is drift in the TG's 21.4 MHz VCXO, and the 85662A's down/up converter 18.4 MHz XO, and the drift in the 3 MHz crystal filters.
Ed
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Re: Yet another TG project
Now I'll talk about the methods used for processing the LO signals. There are many ways to do it, but I used particular arrangements depending on what I had on hand in the junk parts department. My rule of thumb for TG system invisibility is to acquire the signals and get them to appropriate levels, while providing about 100 dB or more of reverse isolation between all activity in the TG, and the SA's LO sources.
The first LO (1LO) is fairly strong, at around 6-8 dBm, so I went without amplification, and used a cascade of one-octave 2-4 GHz isolators. Four of these, followed by the original 2-3.8 GHz BPF from the 8444A, followed by one more isolator, provides 1LO for the output mixer.
2LO is fairly small, around -26 dBm, so needs amplification. Three narrow band isolators, tested for good performance at 1.75 GHz, are used in this chain. First, an isolator, then a 30 dBg amplifier, then another isolator, then a 1.75 GHz BPF, then another isolator, provide 2LO for the IF mixer.
3LO is also fairly small, around -26 dBm - I picked off only a small amount from the 3LO module in the 8568, to minimize loading and interference with the signal that goes to the PLL synthesizer in there. This is the part I discovered late in the game - the need for 3LO rather than a fixed 301.4 MHz source.
It turned out that I had some big old RF modules from an AIL brand synthesizer I junked out twenty years ago, and had only kept these parts intact all this time because they had nice BPFs built in. The BPFs were the wrong frequencies, but close enough to be readily re-tuned for what I needed. The first module is an amplifier/filter/amplifier, and was ready to use pretty much as-is, by re-tuning the BPF and peaking circuits in the amplifier stages for 280 MHz operation. One drawback is that these modules were built for -12V power, but the 8444A PS only has -10V, and I didn't want to make extra power supplies. They work OK on -10V, but have less gain available. They can easily be modified if necessary, for more gain.
The second module was a mixer/amplifier/filter/amplifier/mixer - the exact function I needed, except for the second mixer, which I deleted. I tweaked it up for 301.4 MHz, and added a PIN circuit and a gain stage at the output.
So, the first module amplifies and filters the 280 MHz 3LO, which is then mixed in the second module with 21.4 MHz from a VTXO. The resulting 301.4 MHz is amplified and filtered, then amplified some more. The PIN stage provides gain control for the ALC.
The 301.4 MHz goes to the IF mixer, along with 2LO, to make 2050 MHz, which goes through an isolator, then a 2.05 GHz BPF, and another isolator, then to the output mixer, where it gets subtracted from 1LO to make the center frequency.
You can probably tell by now that I like using isolators, and have a pretty good stock of them. If I didn't, I would not choose to go this route, because to acquire them new would be very expensive, as with most packaged microwave parts. These functions and high isolation can all be readily attained by using lots of attenuation and lots of amplification, properly distributed, in a number of arrangements. It's also likely that so much isolation is unnecessary, depending on filters, amplifier and mixer characteristics, and signal levels - I've probably overdone it by 30-40 dB in some spots, but it's expedient to just have lots. With experimentation, the various sensitivities of the LO ports can be determined for all operating conditions, and just enough isolation can be applied. Even with little isolation, it doesn't hurt anything - it just limits the available dynamic range.
Next time I'll talk about the output section, low frequency limitations of the original output amplifier and mixer, and the low-band amplifier I had to build to reach nearly DC.
Ed
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Re: Yet another TG project
Ed,
How versed are you in the freq. tuning scheme of the 8568? Are you aware that the scheme involved toggling frequency shifts between the 1st and 2nd L.O.’s every 20MHz of freq. sweep? This was done to keep mixing products out of the 21.4MHz IF path.
I have used the 8568B and 8444A opt 059 in 10Hz and 30Hz RBW’s in very narrow spans or zero span with 5 - 20 min. stability, but only after 24 hr. warmup/runtime. The HP-Agilent TG solution was a different box with both freq. and sweep lock circuitry., which the 8444A opt 059 did not have.
I wish you great success with your 8568 TG.
Don Bitters
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Re: Yet another TG project
Ed,
I just recalled a weird intermittent failure involving the 85662A A3A4 and A3A8 brds. The sweep ramp goes to both brds. if I recall correctly. The A3A8 digitizes the sweep ramp and when it hits the proper value it creates an EOS ( End Of Sweep) signal that then goes to A3A4 and the 8568 A22 brd (8566 A16brd). If the EOS is sent early then the Sweep is too short. If the EOS is late or not sent then you only have one sweep and no reset of the sweep. I have replace many 1826-0448 DAC’s for low order bit failures, if I recall correctly the replacement DAC may be 1826-0698?
The DAC’s were id’d as MSB and LSB ramp, sweep, sweep time, and EOS ramp DAC’s.
Also the failure symptoms were similar for too short of a sweep on the 8566 and the 8568, but drastically different between the 8566 and 8566 for the too long of a sweep.
Best regards,
Don Bitters
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