Simple airband VHF fully squelched - UK Vintage Radio Repair and Restoration Discussion Forum

rubicon · 18th Mar 2023, 3:00 am

Have built a few of these, they perform well.
TR1 is the standard superregenerative detector. The tight bias helps keep this stage stable despite the large tuning span, normalising the squelch level.
The 2k7/.01 emitter combo set the quench at around 30kc.
Audio passes to TR2 buffer via a passive LPF. This removes the quench freq.
IC1 a, LM358 is a gain stage giving a low Z output to prevent the volume and squelch controls interacting.
The audio signal is processed by IC1b, a multiple feedback BPF. This removes any audio modulation leaving a band of "hiss" frequencies centered at around 12kc, preventing any audio modulation or residual quench upsetting the squelch gate.
The output is rectified and switches TR3, in turn reverse biasing the audio gating diode, thus muting the audio.
Audio output is via the ubiqutous LM386.

Noise squelch relies on the rapid quieting effect of the SR when it detects a signal and is a reliable and robust way to squelch any radio.
This design has a MDS of 110dBm, a squelch sensitivity of -112 dBm, yes, it will detect a carrier under noise...
hen operating correctly, the squelch will "spit" purely on noise.
Purists will note the lack of any RF buffering, this is not needed.
The maximum hash level eminating from this RX is 100pW, this being swept, or "combed" over a few 100kcs, a form of frequency hopping at the quench freq if you like. A fixed airband receiver with a 12.5kc band spacing will see only a very very small part of this, in fact, my Yupiteru MVT 7200 is struggling to hear this at 3 meters distant.
Performance of this one is very good, it will sit on a frequency for weeks without any need to adjust tuning or squelch. Coverage is 110 to 145 megs, this will vary depending on construction. I use a daughter board to shorten front end connections.

Bazz4CQJ · 18th Mar 2023, 3:34 am

Looks interesting, but the picture of the cct is very hard to see. Any chance you trim it down to size and then scan it as PDF?

B

rubicon · 19th Mar 2023, 2:01 am

Yes, I am trying to lay my hands on a scanner, that will help me.

18th Mar 2023, 3:00 am	#1
rubicon Triode Join Date: Mar 2017 Location: Greymouth, New Zealand. Posts: 17	Simple airband VHF fully squelched Have built a few of these, they perform well. TR1 is the standard superregenerative detector. The tight bias helps keep this stage stable despite the large tuning span, normalising the squelch level. The 2k7/.01 emitter combo set the quench at around 30kc. Audio passes to TR2 buffer via a passive LPF. This removes the quench freq. IC1 a, LM358 is a gain stage giving a low Z output to prevent the volume and squelch controls interacting. The audio signal is processed by IC1b, a multiple feedback BPF. This removes any audio modulation leaving a band of "hiss" frequencies centered at around 12kc, preventing any audio modulation or residual quench upsetting the squelch gate. The output is rectified and switches TR3, in turn reverse biasing the audio gating diode, thus muting the audio. Audio output is via the ubiqutous LM386. Noise squelch relies on the rapid quieting effect of the SR when it detects a signal and is a reliable and robust way to squelch any radio. This design has a MDS of 110dBm, a squelch sensitivity of -112 dBm, yes, it will detect a carrier under noise... hen operating correctly, the squelch will "spit" purely on noise. Purists will note the lack of any RF buffering, this is not needed. The maximum hash level eminating from this RX is 100pW, this being swept, or "combed" over a few 100kcs, a form of frequency hopping at the quench freq if you like. A fixed airband receiver with a 12.5kc band spacing will see only a very very small part of this, in fact, my Yupiteru MVT 7200 is struggling to hear this at 3 meters distant. Performance of this one is very good, it will sit on a frequency for weeks without any need to adjust tuning or squelch. Coverage is 110 to 145 megs, this will vary depending on construction. I use a daughter board to shorten front end connections. Attached Thumbnails

18th Mar 2023, 3:34 am	#2
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,455	Re: Simple airband VHF fully squelched Looks interesting, but the picture of the cct is very hard to see. Any chance you trim it down to size and then scan it as PDF? B __________________ Saturn V had 6 millions pounds of fuel. It would take thirty thousand strong men to lift it an inch.

19th Mar 2023, 2:01 am	#3
rubicon Triode Join Date: Mar 2017 Location: Greymouth, New Zealand. Posts: 17	Re: Simple airband VHF fully squelched Yes, I am trying to lay my hands on a scanner, that will help me.