I built the transmitter almost 20 years ago. It is in the larger box, which originally housed a Heathkit DX-40. There is a lot of soul in that old machine. Details on this construction project are here: https://soldersmoke.blogspot.com/2021/12/junk-box-sideband-from-azores-2004-qst.html (The smaller box is a Barebones Superhet receiver set up for 17 meters.) In the 2004 QST article I discuss a problem I had with "spotting" or "netting." This is something of a lost art, something that you had to do back in the pre-transceiver days, when running a separate transmitter and receiver. This was how you got the transmitter on the receiver's frequency. Essentially you would turn on the carrier oscillator and the VFO and let a little signal get out, enough to allow you to tune the VFO until you heard zero beat on the receiver. My problem was that around one particular frequency, I would hear several zero-beats. This made netting the receiver and the transmitter hard to do.
Important note: This is really just a problem with the "netting" or "spotting" procedure -- the problematic spur does not show up in any significant way in the output of the transmitter. I can't see it on my TinySA. But it is strong enough to be heard in the unmuted receiver sitting right next to the transmitter. And that creates the netting problem.
In the QST article, I said that I noticed that the problem seemed to be centered around 18.116 MHz. As I approached this frequency, the tones -- desired and unwanted -- seemed to converge. That was an important clue. In the article I said I thought that I could eliminate the problem with just one trimmer cap to ground in the carrier oscillator, but looking back I don't think that this really fixed the problem.
I recently took a fresh look at it. Exactly which frequencies were causing the unwanted signals that appeared in my receiver?
I used an Excel Spread sheet to find the culprits.
The first column shows the carrier oscillator and its harmonics. The second column shows the VFO when tuned for a signal at 18.11668 MHz (23.2927-5.17602), along with its harmonics. Check out the 10th harmonic of the carrier oscillator and the third harmonic of the VFO: 69.8781-51.7602 = 18.1179. Those two harmonics would produce the problem I had been experiencing.
I turned to one of Wes Hayward's programs for confirmation. Spurtune08 came in the EMRFD software package. Here is what I saw when I plugged in the above frequencies:
You can see the little spur off to the left of the main signal. In the program, as I tune the 23 MHz VFO frequency, the spur moves closer to the main frequency as I approach 18.116 MHz, just as it does in the real rig. Note that I have only turned on the 10th harmonic of the carrier oscillator and the 3rd harmonic of the VFO. Spurtune08 is very useful. Thanks Wes!
So, what is to be done? For now, I am just restricting my operations on 17 meters to above 18.120 MHz. (I worked several DX stations with it on December 27.) But obviously I need to fix this. This rig needs an exorcism. I think I only need to get rid of one of the harmonics, and the 10th harmonic of the carrier oscillator seems easier to kill. I'm thinking of putting the carrier oscillator in an Altoids box, and then adding some filters to knock down the 10th harmonic.
Here is the G3YCC schematic that inspired this rig. I used G3YCC's carrier oscillator and balanced modulator circuits, just using a 5.176 MHz crystal and changing the tank circuit in the collector:
How would you folks knock down that 10th harmonic?
Posts
A quick and dirty fix that might work - or at least prove you're on the right track - could be just a series tuned trap on the offending harmonic across the carrier osc output. A trimmer with a max capacitance of 30 or 50 pF and maybe 5 to 10 turns of wire wrapped around a pen should be about right.
ReplyDeleteQuickest thoughts:
ReplyDeleteRedesign LO
[1] Choose a LO that produces less overtones: e.g. Colpitts where 1 half of the crystal
gets AC grounded via a cap or a series L - C
[2] After LO, lightly couple an untuned buffer amp to drive bal. modulator. e.g. Common base with untuned (broadband)collector; or if using tuned collector tank, shunt a low K resistor to drop the tank Q.
and/or
[3] low pass filtration of LO and/or VFO
In a NE602 (which seems to be the mixer used in your transmitter) the level of spurs at the output increases dramatically if you do not pay enough attention to the level of IF/VFO signals. I would try using a looser coupling (gimmick cap?) to the VFO and see what happens to the output spectrum of the mixer
ReplyDeletebest 73/GL
Henk/PA0EME
I had the pleasure of a contact via this rig in 2002. I was very suprised to receive my green stamp reurned with the inscription "this dollar bill has travelled from ZL to the Azores and return"
ReplyDeletevry 73 Bill best DX
Peter
FB Peter! Here is the log entry for April 13, 2002:
ReplyDelete13 April 2002
0556 17S HP2CQB 59 Jose
0626 17S WE4H 57 Bradenton Fla. Goerge
0807 17S ZL1PWD 57 Peter N. Island
0819 17S SM0OWX 57 Chris Says SSB Rig Sounds Good
1132 17S VE3OWV 55 Nick 30 mil W of Ottawa HB Amp, also runs HB 6 meter XCVR and DX-100 Also working on HW-100 21.425 AM
20 CU2AAL Ernesto told me that signal is fine after I told him that he should drop…?
My cleanest homebrew receiver has high side VFO, as is yours. A buffered and low pass filtered BFO -- entirely enclosed and with bypassed DC supply line, inside a rail modellers brass box. The VFO is not low pass filtered but well away from the rest of the receiver. (This is NOT one of my compact portable rigs!).
ReplyDeleteThe other ideas also sound good. I like reducing the LO level into the NE602, you don't need much pF there!
Best of experimenter's luck to you. Paul VK3HN.