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Re: Building a simple SA resolution filter
#spectrum_analyzer
¿ªÔÆÌåÓýWhat¡¯s really cool about this is that you can replace the capacitor (to ground between the crystals) with a varactor diode and make the filter center frequency adjustable by using a voltage.? Put a trimmer in series with it so that you can set the midpoint range (at midpoint voltage).? They make great adjustable band-pass filters. ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch ¨C K9ZC Staunton, Illinois ? Owner ¨C Operator Villa Grand Piton ¨C J68HZ Soufriere, St. Lucia W.I. Rent it: Like us on Facebook! ? Moderator ¨C North American QRO Group at Groups.IO. ? email:? bill@... ? ? From: [email protected] [mailto:[email protected]] On Behalf Of erik@... ? The narrowest resolution filter of my spectrum analyzer did not perform as expected to I decided to build a new filter. |
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Building a simple SA resolution filter
#spectrum_analyzer
The narrowest resolution filter of my spectrum analyzer did not perform as expected to I decided to build a new filter.
As I did not want to buy many crystals and go through all the difficult sorting, matching and calculations I decided to go for some crystal filters rather cheaply available on ebay. In contrast with a receiver a SA resolution filter should not be as steep as possible otherwise you may miss some signal easily when you are using a too large frequency range The NDK 10F7.5A looked suitable so I bought some. Measuring them on my VNA they al seem to be on the same center frequency (10.7MHz) which is nice! The input impedance is, according to the datasheet, 1.5kOhm/5pF so using the online matching calculator the matching circuit should be something like this. The 5pF of the crystal should be subtracted from the calculated value of C1 to get the actual C1 In order to confirm the matching circuit I mounted one of the filters on my universal test jig, connected the VNA and connected a tunable inductor and capacitor of about the correct value. This simple setup lets you tune all components till you get the right performance. After some fiddling the polar input impedance chart looked like this Tuning could still be a bit better but the filter loss is very acceptable As I had more of these filters the obvious next step is to use more then one. Two connected directly in series with the impedance matching at the input and output of the whole filter I got a rather disappointing result. Way to wide, not symmetrical and too much loss But then I remembered about connecting a small capacitor to ground in the middle of the filter And using this ancient variable capacitor I was able to tune the filter Next step is adding one or more extra stages, hope for further decreases in bandwidth, see if the loss remains in the order of -2dB and add some shielding |
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Mirros, spurs and harmonics in a Spectrum Analyzer
#spectrum_analyzer
While analyzing the quality of a signal generator the SA shows a number of components next to the base frequency at 6.18MHz..
There are multiple causes for these components. The first obvious are harmonics generated either by the generator or internally in the SA. A second cause is the generation of unwanted mixer products from the various LO's in the SA. A third cause are mirrors where the quality of the IF filters is insufficient to suppress the opposite mixer output. A real life example is this measurement Which of the signals are real? A common way to reduce spurs and mirrors is to wobble the intermediate frequencies of the SA and use exponential averaging to smear the energy of the unwanted signal over a wider range. As you can see enabling this form of spur reduction does have some impact. The signals a 46MHz and 10MHz are almost gone The IMD2 and IMD3 measurements at 12.33MHz and 18.48MHz remain at -42dB and -53dB but how to be sure these are from signal generator and not generated in the SA? The simplest way to check is to enable some attenuation. Adding -10dB again changes the picture. The noise floor moves up 10dB. Most harmonics did go down as is reflected in the IMD2 and IMD3 measurement so the SA did generate most of the harmonics. A further increase of the attenuation does not change the IMD2 and IMD3 so we can be fairly sure we are now seeing the real content of the signal from the signal generator. The peak at 42.83MHz should be at 43.26 (=7*6.18MHz) to be a harmonic. In fact it is not from the signal generator but from the PC keyboard. |
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Phase noise and the choice of the first IF in a spectrum analyzer
#spectrum_analyzer
This may be a difficult and less relevant topic for many of you but writing this post helps me to understand. If this is too much for this group do tell me.
Many of you may have heard about "phase noise" but do you thoroughly understand what this is all about. I also was not aware of the relevance before I started measuring the performance of the my home build spectrum analyzer. The 10.7MHz resolution filter (third IF filter) I'm using? has a -50dB width of about 60KHz and a -90dB width of 100kHz when measured on a VNA. When sweeping this third IF filter in the SA while using a first IF at 2.6GHz a very different filter picture appears The staircase at the center is caused by the discrete steps of the fractional PLL used for the sweep From 20kHz offset and -40dB down there are side skirts and even a shoulder at 150kHz from center. (the peak at 120kHz is leakage) where neither the side skirts or the shoulders are visible on the VNA. These skirts and shoulders are caused by the phase noise of the LO's. Not all energy is in the single intended output frequency but there is also noise generated that reduces when farther away from the intended frequency. A standard way to measure this phase noise is to remove the first mixer and use the first LO as test signal and scan this LO and use a log frequency scale as can be seen in below plot What you see is a upper side band scan, the lower side band scan should and actually does looks the same (apart from the small leakage peak) The horizontal scale is the frequency in MHz from the the LO frequency. The sweep of the first LO is still done lineair so the lower frequencies have less measurement points compare to the higher frequencies. The first point at 0.01MHz is the full LO signal normalized at 0dB and the first point with offset is at 0.1MHz away from the LO. You can not see the resolution filter details (as can be seen in the first picture in this post) as there are insufficient points in this scan at low frequencies but the noise fall-of when further away from the LO signal is clearly visible till about 10MHz where the phase noise goes below the SA noise floor of -105dB A practical implication of this phase noise is when you have a strong (0dB) signal 300kHz away from a weak signal the noise floor of the SA will increase from -105dB (right part of scan) to -80dB so the sensitivity of the SA is reduced in the near presence of strong signals. Do keep in mind that what you see is actually a result of 3 LO's? (first IF at 2.6GHz, second IF at 110Mhz and third IF at 10.7MHz)? so you can not simply attribute all to one LO but the bandwidth of the first and second IF will impact contribution of the second and third LO. This still needs more investigation. Now what has this to do with the choice of the first IF of the SA? Phase noise is caused by noise in the steering of the VCO in the PLL of the LO. If you have a high first IF you need a high output frequency from the LO, in this case of a ADF4351 and a first IF at 2.6GHz? no output dividers are being used. When using a lower first IF frequency (say 110MHz) and using the same ADF4351 the output divider will be 16 and this will reduce the phase noise. To check this I measured the close phase noise when a first IF of 110MHz is used and you get below picture (first IF at 110MHz and second IF at 10.7MHz and no third IF) The shoulders have moved down with about 25dB, still not as good as the VNA measurement. The VNA measurement was done at 10.7Mhz so phase noise is expected to have less impact in the VNA measurement ? But what about the far out phase noise? As you can see in below graph the far out phase noise also has gone down. The divide by 16 of the output of the PLL has increased the steepness of the fall-of of the phase noise. The phase noise at an offset of 300kHz is at the level of the noise floor so a strong (0dB) signal 300kHz way from a weak signal will have no impact on the noise floor of the SA when using the first IF of 110MHz instead of 2.6GHz All this implies when building your own SA you should not blindly go for the highest possible first IF. You have to understand the impact of the PLL in the LO's you use and their phase noise and the output divider in relation to the selected IF frequencies. In general having a high first IF will introduce more phase noise and this makes your SA less sensitive in the presence of strong signals. Its probably better to choose the first IF low enough for most measurements and use a down converter for the odd measurement where you have to go higher. |
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Re: #Jumper_based_Attenuator
#jumper_based_attenuator
Alan More good information.? Thanks. Not needing to go into the VHF range does seem to make things a bit easier.? I will try DS PCB initially to see how it performs.? I have blank stock in several substrate thicknesses.? Now you have got me thinking about more automation than was the original intent.? Using small relays in place of jumpers does sound interesting and could allow tests to be ran based on signal levels found.? Of course distributed capacitance and inductance in the relays would be present, but probably not of much consequence in LF and HF testing.? Might even be able to use MOSFET switches for some of the switch-to-ground functions. ? Initial design will probably focus on relatively narrow sweep ranges as needed for crystal filters and single-band passband filters.? Arv _._ On Mon, Apr 1, 2019 at 1:07 PM Alan <g8lco1@...> wrote:
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Re: #Jumper_based_Attenuator
#jumper_based_attenuator
¿ªÔÆÌåÓýHi Arv A tracking generator can be made with say 1x 10dB and 2x 20dB pads with a variable 0-10dB control? which gives a 60dB range. I have used TG¡¯s for a very long time, if I wanted to go to very low levels ( which is pretty rare ) I would prefer to add some external pads to get down to very low levels. But most of the time sweeping amplifiers, filters and mixers you are around 0 to -40 dBm. ? The TC is normally made by allowing a little compression in the LO drive to the mixer so that the level changes (that are almost inevitable in sweeping? ) ?are lost in the slight limiting action. A DBM works nicely when the LO port is driven quite hard.? But you can attenuate the RF port drive by either using a pot? directly between the LO and the DBM or by using a tuned amplifier and varying the gain. If you wanted to be real smart then rectify the tuned amp output and compare that against a voltage from a pot in an integrator that drives the buffer amp. ? I do feel that you might get fed up swapping links around and they are difficult to screen which may cause stray signals from local broadcasters to creep in and cause confusion.? There are some cheap and tiny DPDT relays that cost very little, might be inclined to make a? PCB with pads switched by relays on the basis that if one messes you around it gets dumped and another cheap clone takes it¡¯s place. ?Then you can have a 6 way switch to control the relays? with 0-10-20-30-40-50 dB clicks . ? This is a variation on the ¡°Law of Sod¡¯s¡± , if you have a spare or two it will never get used ( unless you loose the spares!!!!!). But if you don¡¯t have a spare then you will need it at a critical moment. ? If you have a 0-50dB module? with 1206 surface mount R¡¯s on the solder side of a DS PCB then you should be OK up to around 100MHz with the normal ?50 ohm tracks of 2.7mm on 1/16¡± board.? Make a few and you should be all set. ? ( I would use a diode across every relay coil and decouple the coil connections with -say 100nF to board ground to stop leakage) ? Regards, ? Alan G8LCO Sent from for Windows 10 ?
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Re: Ebay green and black ADF4351 module performance
Using Arta (still learning how to use) to do the FFT and some patience the differences between the modules becomes better visible
Same setup using dbm, TCXCO to LO port, DUT to RF port and USB audio input to IF port. LO on 50MHz, DUT on 50.01MHz FFT window (Kaiser4?) selected for the least leakage First the green module with the TCXCO (with exponential averaging) The width at -100dB is about 150Hz, 50/100Hz spurs assumed to be from the TCXCO Then a black module and the TCXCO The width at -100dB is somewhat smaller, around 100Hz And last two green modules, unfortunately I forgot to adjust to the same RMS level The 50/100Hz spurs from the TCXCO are (almost?) gone, width at (-100 +4)dB seems equal to green module with TCXCO. Should the width be larger if the TCXCO had no noise? Can I have some feedback on the measurement approach and conclusions? |
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Re: #Jumper_based_Attenuator
#jumper_based_attenuator
¿ªÔÆÌåÓýHi Arv, ? Very well made points, pins and links are very cheap and ?there is a good wiping action. The plating is sometimes ?the thinnest possible because of the cost so it might not last but quick and cheap to swap out. ? It is not that easy to find cheap AND good switches unless you ?go ?for a vey good brand that don¡¯t put out production offshore. ?All depends on your top frequency.? An RF attenuator is normally designed around a coaxial transmission line , the line from input socket to the output is the inner line while the box walls are the outer line. If you can keep the ratio of the sizes to 50 ohm all the way through? ( including the switches and pads) and the resistors don¡¯t mess up the line too much then you have something good at VHF and beyond. In effect you make the stray inductance and capacitance part of the line so it disappears. ? That¡¯s why a good attenuator is expensive, you put metal around the switches to turn each switch into parts of a 50 ohm line! An other approach is to use a pcb transmission line on one side of a double sided board with toggle switches in the 50 ohm line to insert or bypass a pie pad. That is widely used commercially and OK up to 500MHz. ? Switches are always the parts that fail ( unless it¡¯s been damaged by running excessive power ), nearly every secondhand attenuator is likely to have a noisy switch or a pad with odd attenuation? in my experience. Folks just don¡¯t give away good ones. ? Real friends have CNC milling machines so they machine out cavities in a aluminium bar to take pcb attenuators! You mount the switches in the block then fit the board then tighten the nuts so that everything fits before the switch spills are soldered in. Good idea to fit oversize endplates so that WHEN you drop it the switch levers are not smashed in. ? Take it that the links are used up to around 10MHz.? One problem that crops up with low pass filters that stop working is that the inductance of the grounded? parts starts to increase as the frequency goes up. The common ground link? will act as a very small inductor as frequency increases, it may become noticeable above 10MHz or so with higher value pads. If you wanted to use links at higher frequencies then use two links to ground each shunt resistor otherwise you get a sneak around path at HF. ? ? Many designers limit maximum attenuation? to 20 dB in RF attenuators ?per switch , this limits the errors at higher frequencies due to stray c in the series elements and the stray L in the shunt elements.? Modern Chip resistors can be good to GHz!?? They also can be stacked to get a particular value, it¡¯s often good to use the next highest value then select an additional resistor to parallel onto the value you need. ? But if you don¡¯t go beyond 1 MHz then you can get away with? wire ended regular metal films , they are very stable and withstand overload very well provided that you don¡¯t have spiralled resistors ( resistors where a spiral cut is made into the resistor to greatly increase the resistance. ? An other option is to use a solid state FET switched attenuator, these often go? 0 -31,5 dB in 0.5 dB steps and are good to around 3GHz. ? ? Good Luck Alan G8LCO ? Sent from for Windows 10 ? From: Arv Evans
Sent: 29 March 2019 18:55 To: [email protected] Subject: [HBTE] #Jumper_based_Attenuator ? Hello ? Earlier post regarding DC analysis of step attenuators was in relationship to design and building jumper-based attenuators.? ? ? In the past I have built step attenuators using DPDT slide switches and DPDT rocker switches, but over time and with frequent use these usually develop Instabilities, especially if the switches are of low quality.? Present approach is to use simple jumpers in place of the switches.? Prototype build and testing seems to indicate that these may be better for long-term use on the test bench.?? ? ? Arv _._ ? ? ? ? ? |
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Re: Ebay green and black ADF4351 module performance
On Sun, Mar 31, 2019 at 10:32 AM, <erik@...> wrote:
Thanks for the pointers to the very low noise amplifiers but at -110dB, is there still much to gain? I see more now... I wasn't sure what the noise floor of the UMC202HD is. Neat to see the images and data, thanks for sharing and all the info.? In regards to the filtering method to reduce the USB noise... would similar like was done with this modification help to reduce the ripple? .? I'm not sure the math offhand... though I am sure can be calculated somehow to get the ripple reduced (from clipping?) from the power supply I guess with my limited experience and seeing how to clean up an AC to DC rectified signal.? That's why I referenced this link also since might provide some ideas if not known already since shows clipped signal not smoothed out and ideas for ripple reduction:? |
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Re: Ebay green and black ADF4351 module performance
First real measurement results
Using a UMC202HD (24bit 192kHz USB audio input) connected to the IF port of an ADE-11X passive dbm using an isolation transformer to avoid ground loops. A TCXCO on 10MHz is connected to the LO port. The TCXCO needs to warm for at least 5 minutes because the heater draws enough current to cause 50Hz residu pollution? The module to measure is set to 50.010MHz?and connected to the RF port Noise level when nothing connected to the RF port The frequency is on top in kHz, 100Hz per tick Noise level is average at -110, max input signal is 0dB so 110dB(?) dynamic range When using a black module I measure The signal top is at -6dB. Width at -80dB is about 100Hz Many small spurs start at -85dB and disappear into the noise at 500Hz offset. They seem to be at a 100Hz grid. This could imply the spurs are caused by some residual 100Hz ripply of one of the analogue supplies I am using. The noise floor also shows the 100Hz grid in the noise. Need to investigate. The green module has similar spurs but they are less visible because of somewhat more noise in between Beyond 500Hz offset there is no visible noise difference but the black module has all kind of spurs. I modified one green module to isolate and decouple all supply lines with inductors. There seems to be no difference in the close phase noise and beyond 500Hz offset again everything disappears in the noise floor Notice the frequency offset, the TCXCO is helpful to calibrate the modules Need to search for an audio FFT with more resolution, hope that will also reduce the noise floor. Thanks for the pointers to the very low noise amplifiers but at -110dB, is there still much to gain???? |
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Re: Ebay green and black ADF4351 module performance
I'm not sure if this will help in regards to the low noise amplifier situation... however..., I have a SB0270 I've used and I need to verify performance compared to the UMC202HD that I haven't used yet.? There is some data I've found online and looks like the SB0270 performs better in regards to having a lower noise floor.? I found a great deal for a SB1095 which has an even lower noise floor and am planning to clean up per this method, which lowers the noise floor a little more also:
Interesting is there is some profile performance comparison data in a different user applications though does graph some sound card performance:
Recently, I found the INA114 which has a low drift and like the OXCO's having lower drift short term maybe, being better short term for phase noise reduction say reading recently compared to a GPSDO that is more accurate long term though might not be better for phase noise due to short term drift, there is something value added.? I am not sure the later observations that are more related to the Lock-In Amplifier projects observations like in these links: ?
Hope I'm not off in the wrong direction... figured I'd reference at least some others work that might inspire some ideas to improve performance.? |
K9HZ
jafinch78 .