21-12-2016, 11:37 PM
Greetings all,
I won't bore the membership with the background on this but it is kind of someone else's home brew circuit for a vhf front end of a pulse counting FM radio, comprising an aperiodic amp and autodyne converter, for the most part the set's performance is excellent despite its simplicity. The design looks like it owes its origins to the likes of the Sinclair micro FM.
The problem with it is that tuning towards the lower end of the band causes the set to produce a motorboating type of noise that I'm pretty sure I've tracked down to a parasitic oscillation at hf causing the biasing of the autodyne converter to oscillate at an audible frequency. The frequency of the parasitic oscillation is at least four times the intended in band signal.
The transistors are both Fairchild ss9018, the two rf amp chokes are 15uH and the oscillator coil is 200nH. The chokes and coil are all at 90° to each other so interaction between them is unlikely. the rest is laid out with good rf hygiene.
I'm not a great fan of the autodyne converter but I'm restricted to using this circuit as near original as possible as a ready made PCB is involved.
Any suggestions for a "patch" to get it working would be appreciated as I'm now at a bit of a loss.
Cheers, Andy.
I won't bore the membership with the background on this but it is kind of someone else's home brew circuit for a vhf front end of a pulse counting FM radio, comprising an aperiodic amp and autodyne converter, for the most part the set's performance is excellent despite its simplicity. The design looks like it owes its origins to the likes of the Sinclair micro FM.
The problem with it is that tuning towards the lower end of the band causes the set to produce a motorboating type of noise that I'm pretty sure I've tracked down to a parasitic oscillation at hf causing the biasing of the autodyne converter to oscillate at an audible frequency. The frequency of the parasitic oscillation is at least four times the intended in band signal.
The transistors are both Fairchild ss9018, the two rf amp chokes are 15uH and the oscillator coil is 200nH. The chokes and coil are all at 90° to each other so interaction between them is unlikely. the rest is laid out with good rf hygiene.
I'm not a great fan of the autodyne converter but I'm restricted to using this circuit as near original as possible as a ready made PCB is involved.
Any suggestions for a "patch" to get it working would be appreciated as I'm now at a bit of a loss.
Cheers, Andy.
