I got the two diode, one transformer product detector working well, but with it a new problem arose: 455 kHz energy from the BFO was leaking past the product detector back into the S-meter/AGC circuitry. This showed up in the form of a constant S-3 reading when I switched to SSB/CW. This was annoying.
I figured the problem was that the only signal really being balanced out was the IF signal going into L1 of the product detector. I took another shot at putting the BFO signal into this port, with the IF signal going into the unbalanced potentiometer port. This did indeed take care of the BFO leakage S-meter problem, but once again the SSB did not sound great -- I think the old problem of simultaneous envelope and product detection returned.
This was obviously a port isolation problem. I remembered that the diode ring "doubly balanced" configuration has much better port isolation. So on Sunday morning I built one, first in LTSpice and then on the bench.
For the bench model I used some PC board pads out of Pete Juliano's $250,000 CNC machine. For the toroids I used two trifilar coils wound by Farhan's dedicated staff in Hyderabad. The diodes were sent to me by Jim W8NSA. So there was lots of soul in this new machine.
The circuit worked in LT Spice and at worked well when tested on my bench with my FeelTech (for the BFO) and HP8640B (for the IF signal) sig gens with my Rigol 'scope watching for the audio out.
But I ran into some problems when I popped the new board in there in place of the old product detector: The 455 kc BFO leakage problem is gone and the S-meter is where it should be, but...
-- I'm seeing a return of the old simultaneous envelope and product detection problem. SSB was sounding scratchy again and indeed, when I removed the BFO signal from the diode ring circuit I could hear SSB signals making it into the audio amplifiers. These signals sounded just like AM signals as heard through an envelope detector without a BFO.
-- The diode ring circuit also had a very bad effect on how the HA-600A worked in AM mode. It seemed like the new circuit was loading down the diode AM demodulator. SW broadcast signals sounded awful in the AM mode until I disconnected the IF input to the diode ring circuit (this input is NOT switched -- it is always connected, even in the AM mode).
So, for now, am back to using the two-diode, single transformer, singly balanced product detector with IF signal going to the balanced (L1) port and the BFO going in through the wiper of the 100 ohm pot.
Any suggestions on how to overcome the problems with the diode ring circuit?
The diode ring wants to be fed from a 50ohm source. The best will be to use a long tailed pair and separate the two detectors. See W7ZOI progressive receiver of 1981.
ReplyDeleteHell Bill,
ReplyDeletewhat you describe sounds to me like the level coming from the IF is so strong it's enough to turn on the diodes in the DBM. That would explain the "loading down" (nonlinear impedance of the diodes causing amplitude distortion on the IF) and the "signals heard through the mixer". And didn't you mention it before, it was strong enough to cause problems in the other mixer as well? You could try to put two diodes in series in place of each diode, leave the BFO disconnected (it will propably not be strong enough now) but you CAN check for the envelope detection and "loading down" of the IF like this! When this is indeed the problem and both problems are now solved, then tackle the BFO amplifier.
Staying with the diagnosis, a different (easier) cure might be to attenuate the IF going into the mixer, without affecting the IF going to the envelope detector too much - something like this http://www.qrp.gr/dircoup/index.htm might work.