|
the 45 mhz if amplifier is distorting and producing a second harmonic at 90 mhz. this mixes with the local oscillator to produce a spur at 90 mhz - local oscillator.
here is how it happens :
the spur moves down as you tune up. when the radio is tuned to 28 mhz, the spur is at 17 mhz. when the ubitx is tuned to 28.150 mhz, the spur is on 16.850 mhz.? at 28 mhz, the local oscillator? is at 73 mhz. there is a signal X such that : X - local oscillator = 17 mhz local oscillator is 73Mhz. X - 73 = 17, X must be 90 MHz.
after checking the spur on a number of frequencies between 15 MHz and 30 MHz, it was confirmed that the above formula works consistently to predict the spur.?
what is th cure? there are three solutions 1. decrease the distortion of the if amp by modifying or replacing the IF amp (the tx side at least) 2. add a low pass or bandpass filter between the final mixer and the if amp. 3. move to 60 mhz or 70 mhz IF insteead of 45 mhz. a double or triple bandpass should do it.
this is a result of joint investigation between Raj and I.
- f
|
Excellent finding! Adding a low pass filter seems the easiest solution. At which impedance should be designed? 50ohm, 500ohm? Where exactly place it, considering the schematics of ver. 3 and ver. 4. I hope it doesn't involve cutting traces on the board.
Il 03/nov/2018 03:39, "Ashhar Farhan" < farhanbox@...> ha scritto: the 45 mhz if amplifier is distorting and producing a second harmonic at 90 mhz. this mixes with the local oscillator to produce a spur at 90 mhz - local oscillator.
here is how it happens :
the spur moves down as you tune up. when the radio is tuned to 28 mhz, the spur is at 17 mhz. when the ubitx is tuned to 28.150 mhz, the spur is on 16.850 mhz.? at 28 mhz, the local oscillator? is at 73 mhz. there is a signal X such that : X - local oscillator = 17 mhz local oscillator is 73Mhz. X - 73 = 17, X must be 90 MHz.
after checking the spur on a number of frequencies between 15 MHz and 30 MHz, it was confirmed that the above formula works consistently to predict the spur.?
what is th cure? there are three solutions 1. decrease the distortion of the if amp by modifying or replacing the IF amp (the tx side at least) 2. add a low pass or bandpass filter between the final mixer and the if amp. 3. move to 60 mhz or 70 mhz IF insteead of 45 mhz. a double or triple bandpass should do it.
this is a result of joint investigation between Raj and I.
- f
|
Which corner frequency for the LPF solution should be used, 50Mhz?
Il 03/nov/2018 08:19, "iz oos" < and2oosiz2@...> ha scritto: Excellent finding! Adding a low pass filter seems the easiest solution. At which impedance should be designed? 50ohm, 500ohm? Where exactly place it, considering the schematics of ver. 3 and ver. 4. I hope it doesn't involve cutting traces on the board.
|
Interesting, I had built the uBITX bi-amp stage only, on a small pcb some time ago. I just measured the 2nd and 3rd harmonics as shown below, with -30, -20 and -10dBm input from an HP8640B sig gen and monitored the output on an SA. The other 'grass' is local fm stations and shack stuff but the 2nd and 3rd harmonics have markers 2 and 3 on them. 45MHz is marker 1.   
|
Glenn,
Please try a mod..
Remove first transistor's base bias resistors uBitx = 1K and
2.2K
Connect a 39K or 47K resistor from first base to last emitter.? The
emitter voltage will be around 5-7v.
I used 44K and voltage was 6.8 half of my supply.
repeat the test below. I want to see if the feedback improves the
distortion in any way.
Cheers & Thanks
Raj
At 03-11-18, you wrote:
toggle quoted message
Show quoted text
Interesting, I had built the
uBITX bi-amp stage only, on a small pcb some time ago.
I just measured the 2nd and 3rd harmonics as shown below, with -30, -20
and -10dBm input from an HP8640B sig gen and monitored the output on an
SA.
The other 'grass' is local fm stations and shack stuff but the 2nd and
3rd harmonics have markers 2 and 3 on them. 45MHz is marker 1.
![[]](/g/BITX20/attachment/61266/)
![[]](/g/BITX20/attachment/61266/1)
![[]](/g/BITX20/attachment/61266/2)
Content-Id: <[email protected]>
X-Attachment-Id: [email protected]
Content-Type: image/jpeg; name="neg30dBm_input.jpg"
Content-Disposition: inline;
filename="neg30dBm_input.jpg"
Content-Id: <[email protected]>
X-Attachment-Id: [email protected]
Content-Type: image/jpeg; name="neg20dBM_input.jpg"
Content-Disposition: inline;
filename="neg20dBM_input.jpg"
Content-Id: <[email protected]>
X-Attachment-Id: [email protected]
Content-Type: image/jpeg; name="neg10dbm input.jpg"
Content-Disposition: inline; filename="neg10dbm
input.jpg"
Content-Id: <[email protected]>
X-Attachment-Id: [email protected]
Content-Type: image/jpeg; name="neg30dBm_input.jpg"
Content-Disposition: inline;
filename="neg30dBm_input.jpg"
Content-Id: <[email protected]>
X-Attachment-Id: [email protected]
Content-Type: image/jpeg; name="neg20dBM_input.jpg"
Content-Disposition: inline;
filename="neg20dBM_input.jpg"
Content-Id: <[email protected]>
X-Attachment-Id: [email protected]
Content-Type: image/jpeg; name="neg10dbm input.jpg"
Content-Disposition: inline; filename="neg10dbm
input.jpg"
Content-Id: <[email protected]>
X-Attachment-Id: [email protected]
Content-Type: image/jpeg; name="neg30dBm_input.jpg"
Content-Disposition: inline;
filename="neg30dBm_input.jpg"
|
Ashar,
I always had a lot of doubts that the first mixer should be the
culprit but suspected taht the first BiDi amp must be the reason
in producing harmonics. But now You and Raj confirmed my
suspicion. This findings also explain why putting another xtal
filter between the output of the BiDI amp (in TX direction) and
mixer.
My proposal: first of all put a diplexer on the IF output of the
mixer and the following stage. THen use two 90 deg hybrids and in
between tho identical xtal fitlers, i.e. split the 45 MHz IF
signal into two path, filter them within two identicla filters
(gain and phase) and combine. Both dipexer and 90 deg hybrids with
the xtal filters will attenuate harmonics.
I am also thinking of another diplexer (LP - Hp ) at the
RF-In/Out of the mixer, the high frequency path be treminated into
50 ohms. In TX mode the image (LO+IF) will then be terminated and
not be reflected into the mixer.?
In addition do a good leveling of all stages in the TX path, but
keep in mind that the balanced modulator has a limited LO
suppression, so the carrier suppression (and image band) must be
taken into account.
Henning Weddig
DK5LV? ?
Am 03.11.2018 um 03:39 schrieb Ashhar
Farhan:
toggle quoted message
Show quoted text
the 45 mhz if amplifier is distorting and
producing a second harmonic at 90 mhz. this mixes with the
local oscillator to produce a spur at 90 mhz - local
oscillator.
here is how it happens :
the spur moves down as you tune up.
when the radio is tuned to 28 mhz, the spur is
at 17 mhz. when the ubitx is tuned to 28.150 mhz, the spur
is on 16.850 mhz.?
at 28 mhz, the local oscillator? is at 73 mhz.
there is a signal X such that :
X - local oscillator = 17 mhz
local oscillator is 73Mhz.
X - 73 = 17, X must be 90 MHz.
after checking the spur on a number of
frequencies between 15 MHz and 30 MHz, it was confirmed that
the above formula works consistently to predict the spur.?
what is th cure?
there are three solutions
1. decrease the distortion of the if amp by
modifying or replacing the IF amp (the tx side at least)
2. add a low pass or bandpass filter between
the final mixer and the if amp.
3. move to 60 mhz or 70 mhz IF insteead of 45
mhz. a double or triple bandpass should do it.
this is a result of joint investigation
between Raj and I.
- f
|
Not much change really Raj, I used 39k.
At -10dBm input, there is a slight improvement by maybe 2-3dB
At -20 & -30dBm in, virtually same result.
I wonder if a notch could be added for the 2nd Harmonic..... Did you do any sums on the 3rd harmonic? It may also contribute.
glenn
|
Raj
The input VSWR with that mod increased the input VSWR to around 7:1.? (measured 0-30MHz, my current setup)
I didn't check before the mod but assume its closer to 1:1.
glenn
|
Glenn,
pleas keep in mind to check the output of Your signal generator,
mostly the ahrmonics are in the same range i.e. about -30 dBc to
the main cyarrier. So to make sure You do not make false
measurements, place a low pass fitler between the output of Your
sig gen and DUT.
Henning
Am 03.11.2018 um 10:33 schrieb Glenn:
toggle quoted message
Show quoted text
Not much change really Raj, I used 39k.
At -10dBm input, there is a slight improvement by maybe 2-3dB
At -20 & -30dBm in, virtually same result.
I wonder if a notch could be added for the 2nd Harmonic..... Did
you do any sums on the 3rd harmonic? It may also contribute.
glenn
|
Thats a given. The stage becomes a high impedance input.
Try a 470 Ohms parallel with input to compensate the resistors
removed.
We did discuss the notch but I have not tried.
After seeing your results I feel the stage is OK for upto -5 dbm
input where as the actual input
is -30 to -40 dbm. The output of the stage is max -10 dbm and so
the input will be 15 -20dbm
less (i.e. gain of stage) So your Bidi amp is good to go.
So where is the offender, it is the strong harmonics of the final that is
getting coupled from
the relay contact loops and into the input of the Bidi amp which is
getting overloaded !
This unholy mix of signals is causing spurs.
Replacing the stock top contact relays with bottom contact AXICOM of Mike
Doty relays reduces
the loop size and so the coupling to L5. I noticed if the stock 45MHz
filter is mounted high by 3mm
then the spurs increase, it needs to be close to the board. That gave me
the clue!
The harmonics are also getting picked up by L1/2/3/4 !
1: Change the relays and you will see major relief. DO NOT mount in
sockets, they add to the loop.
additionally but may not be needed for your board.
2: Change L5/7 will make a small change only if the relay is
changed.
3: Change L1/2/3/4, one of my boards became unstable if any wires or
finger was near.
4: Add another 45MHz filter at the Bidi out to clean up after the Bidi
but with a big power loss.
5: Do't over drive on higher bands. Drive settting that gives 10W on 40M
is fine on all bands
with only relay change. We need a audio clipper / hard limiter for the
next board.
Raj
At 03-11-18, you wrote:
toggle quoted message
Show quoted text
Raj
The input VSWR with that mod increased the input VSWR to around
7:1.? (measured 0-30MHz, my current setup)
I didn't check before the mod but assume its closer to 1:1.
glenn
|
Correction:
After seeing your results I feel the stage is OK for upto -5 dbm
input output where as the actual input
is -30 to -40 dbm. The output of the stage is max -10 dbm and so
the input will be 15 -20dbm
less (i.e. gain of stage) So your Bidi amp is good to go.
At 03-11-18, you wrote:
toggle quoted message
Show quoted text
Raj
The input VSWR with that mod increased the input VSWR to around
7:1.? (measured 0-30MHz, my current setup)
I didn't check before the mod but assume its closer to 1:1.
glenn
|
As the saying goes two heads are better than one, good to hear it is finally cracked.
73 Steve
|
Great you found the likely cause of the 2IF spur.
Now why is the 45mhz IF distorting?
Two things, it needs to be clean to at least -6 or so DBM and how much gain is needed to
get to that from the ouput of the 45mhz crystal filter.
I'd suggest starting with transistors that actually have performance at 45mhz.
Allison
|
Harmonics of the HP8640b are of the order of -50dB at output of the gen upto ~+10dbm..
toggle quoted message
Show quoted text
On Sat, Nov 3, 2018 at 09:43 PM, Henning Weddig wrote:
Glenn,
pleas keep in mind to check the output of Your signal generator, mostly the ahrmonics are in the same range i.e. about -30 dBc to the main cyarrier. So to make sure You do not make false measurements, place a low pass fitler between the output of Your sig gen and DUT.
Henning
Am 03.11.2018 um 10:33 schrieb Glenn:
Not much change really Raj, I used 39k.
At -10dBm input, there is a slight improvement by maybe 2-3dB
At -20 & -30dBm in, virtually same result.
I wonder if a notch could be added for the 2nd Harmonic..... Did you do any sums on the 3rd harmonic? It may also contribute.
glenn
|
Allison, for -6dBm output level? also means the current in the amp needs to be increased also for better liniarity.
toggle quoted message
Show quoted text
On Sun, Nov 4, 2018 at 08:17 AM, ajparent1/KB1GMX wrote:
Great you found the likely cause of the 2IF spur.
Now why is the 45mhz IF distorting?
Two things, it needs to be clean to at least -6 or so DBM and how much gain is needed to
get to that from the ouput of the 45mhz crystal filter.
I'd suggest starting with transistors that actually have performance at 45mhz.
Allison
|
Glen.
Yes that the problem.? Or it could be that the two emitter followers are not following...
Hint that circuit at 12Mhz works ok, at 45mhz no so much.? ?Compound amplifiers
without feedback tend to have issues. and Q21/22 are outside the feedback.
And, HP8640B.... right tool for the task.
Allison
|
Right now the 45MHz amp has a gain of about 17dB at 45Mhz and is 3.3dB down at 90Mhz.
Total current draw in my proto build is 32mA.?? With? 3.1mA ? in the 1st emitter follower and 13.3mA? in the 2nd.
Maybe a high P1dB MMIC could be used to replace that section at the cost of current draw, Tx only.
|
Allison, a Mini-circuits "PHA-1" might do the trick. I don't have any though.
I don't know how you would get on with it switching direction, back to the Rx side in the Bi-Amp arrangement. Somebody here was experimenting with MMIC bi-amp though but not seen any outcome.
|
Will this new discovery require any work in the areas that Allisons power flatten mod provides? changing the 6 3904¡¯s and other related parts. Reason I am asking is I am about to do that mod (easy version) as I have all the parts but if any work in that area needs to be done for addressing these spurs maybe Ill wait to see what solutions are available.?
My guess is one thing shouldn¡¯t have to do with the other and the replacing the 3904¡¯s with 2n2222A should be a good thing anyway.?
Joe
toggle quoted message
Show quoted text
On Nov 3, 2018, at 6:28 PM, Glenn < glennp@...> wrote:
Allison, a Mini-circuits "PHA-1" might do the trick. I don't have any though.
I don't know how you would get on with it switching direction, back to the Rx side in the Bi-Amp arrangement. Somebody here was experimenting with MMIC bi-amp though but not seen any outcome.
|
i used a two section LPF, with a corner freq of 36 mhz. 47pf, 300nh, 100pf, 300nh, 47pf. the lpf had to be mounted away from the mixer to be clean. i dont know why.
- f
Which corner frequency for the LPF solution should be used, 50Mhz?
|
Raj vu2zap