Direct Conversion Experiments
Introduction
Having built a simple Crystal Set and a Super Regenerative receiver I was keen to experiment with a Direct Conversion receiver.
Audio Pre Amp
As the MultiPig+ is a modular radio. I plan to use its VFO and Band Pass Filter for my DC experiments, which will be based on the splendid series of RADCOM articles by Eamon Skelton
Built Eamons DC Audio Pre amp (Radcom Feb 2006), works perfectly, and with a carefully placed damp finger receives Radio 5.
Balanced Diode Ring Mixer
Diodes?
Next, I started building the homebrew diode
ring mixer using N4148 diodes(!) which I measured and matched. Hooking
up the scope and measuring the output from the sig gen, only getting .25 volt
Peak
To Peak this would not be enough to get the diodes to conduct.
Wired up the Multipig+ VFO and got 2 volts PTP. It seems to swamp everything,
and make the PLL go rather mad, so reduced the coupling to the mixer and got
the PTP down to a volt or so. Still just makes horrid loud hum. A few thoughts
so far
- A wiring prob with the "output" torroid.
- RF everywhere(!)
- The PLL unlocking and needing to be disconnected or the cap changed drastically, in order to get it to lock again, RF
- A 2 volt PTP VFO will get everywhere!
Made up some coax connections to enable easy linking of the Multipig+ modules, and to reduce some of the stray RF(!)
Band Pass Filter
Tuned up the Multipig+ BPF by using the Multipig+ VFO as an input, and connecting the scope to the output then peaking up the caps. This seemed to work very well, there being very little loss through the filter.
Checking the Diode Ring Mixer
Checked and rechecked the wiring of the diode ring mixer, it was correct. Despite a long wire "antenna" still had no joy receiving anything! Checked the mixer operation by connecting the signal generator to the input and tuned for a heterodyne, so it does basically work!
SBL1
Alan GM1SXX had kindly sent me some mixers HPF-505x-1 (SBL 1 equivalent).
Swapped the homebrew diode ring mixer for the SBL1, carefully checking the connections while fitting it. Wired it all up and deployed the 40m tuned antenna(!) With some very careful tuning I could just hear some faint CW! with the simple tuning pot with no knob on it the tuning is very sharp indeed. I now suspect the key problem is a lack of audio gain, and or the antenna! The audio amp was designed for this purpose so the next thing is to carefully check the wiring.
Testing the audio amp, connected up the signal generator to the input and tuned for a heterodyne, connected the speaker amp to the output of the first audio transistor, a very quiet tone, then connected to the output of the second transistor, a louder tone. Well at least it is broadly working! next a check of the circuit.
Another Bench
Took the receiver over to Tom G4BYE and connected it up to his wire antenna and signal generator. All worked very well, much better than on my bench.. Great fun tuning the radio with a signal generator!
Had another go back on my bench using the Multipig+ VFO, worked a bit but it was very noisy, with very quiet signals. Plus the 2V PTP from the VFO had now reduced to a very scrappy 1V PTP sine wave.
Again I took everything back to Tom. The basic problem was down to the VFO overdriving the mixer, Tom installed a potential divider to attenuate the input down to approximately .4 V and maintain roughly 50ohm impedence either side of it. Everything started to behave then. All worked surprisingly well considering there was no BPF, no real audio filtering, and only 12ft of wire as an antenna!
Club Dipole
Tried the rig on the club dipole, wall to wall Radio 5.. Until the BPF was connected, then it really lit up with CW.