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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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1st Jul 2020, 2:13 pm | #1 |
Octode
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
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100MHz preamp
My Cossor 524 is working quite well on FM. However when listening to Radio 3 the volume has to be wound up quite high and so the faint hum can become annoying. On other stations the volume is adequate. I am thinking that the signal strength is less than ideal so I have attempted to construct an RF pre amp.
I used the circuit shown. http://electronics-diy.com/electroni...tic.php?id=902 The problem is that it won't amplify. I am injecting a signal at 90MHz and looking at the output on a scope. The scope is only a 50MHz bandwidth but I am comparing the input to output levels not absolute values. Is the circuit ever going to work or have the authors left out a critical component. If it is useless than can any one recommend a pre amp. Malcolm |
1st Jul 2020, 4:16 pm | #2 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
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Re: 100MHz preamp
Two ideas:
1. You need to be sure that the L/C input cct. is actually resonant at the freq. of interest. 2. Have you constructed this pre-amp. with very short leads joining the various components - the usual practice at VHF? Al. |
1st Jul 2020, 4:27 pm | #3 |
Octode
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
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Re: 100MHz preamp
Hi, the input circuit does resonate at the required frequency, in my case about 90MHz. There is a definite peak as the capacitor is moved, there being two peaks as its a continuous rotation trimmer. The leads are all as short as possible, the board I built it on is just a lot of plated through holes so the components are connected on the reverse side.
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1st Jul 2020, 5:15 pm | #4 |
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Re: 100MHz preamp
I don't think RF gain will increase the audio level, it may reduce the noise. Radio 3 is a quiet station to cope with crescendos.
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1st Jul 2020, 7:49 pm | #5 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: 100MHz preamp
Radio3 is well-known for being disturbingly laid-back as far as FM deviation is concerned: so it will generally tend to sound quiet when compared to similar BBC/commercial entertainment-stations who invariably use rather more-optimistic audio processing.
So your receiver may not actually have a problem, the problem may well be with the broadcaster! |
1st Jul 2020, 8:00 pm | #6 |
Octode
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
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Re: 100MHz preamp
Thanks for the comments. I may put the sig gen into it and see what the sensitivity really is. Even if the radio is ok and the preamp won’t make a difference S a technical exert, why doesn’t it work?
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1st Jul 2020, 8:31 pm | #7 |
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Location: Oxford, UK
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Re: 100MHz preamp
I don't know why it doesn't work, but it seems needlessly complex for such a simple requirement. You just need some broadband gain before the radio input - a single VHF transistor, 1M between the collector and base, 4k7 between the collector and power rail, a small cap to feed the input to the base and the output from the collector and Bob's your uncle. You should get about 10-15dB of gain. Simple as they come, minimum component count, easy to understand. Even a jellybean transistor probably still has useful gain at 100MHz.
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1st Jul 2020, 10:03 pm | #8 |
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Re: 100MHz preamp
For reasonable Q and some sort of FET device, the belling lee antenna socket tapping looks a bit too far up the inductor. Somethiing more like a 20% tap may be better, yours looks more like 50%.
David
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1st Jul 2020, 10:40 pm | #9 |
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
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Re: 100MHz preamp
It looks like L2 is trying to be a 16:1 impedance transformer but I think it needs to be tuned to 100MHz with a capacitor across the primary because there won't be enough reactance in the (airspaced) primary to make a wideband 16:1 transformer.
Also, it looks like the 1nF decoupling cap is a long way from the gate of the top transistor and this invites instability problems. The overall layout and component inductance needs to be tiny here to avoid the generation of negative resistance at the drain. However, I think the main problem is to do with L2. Is there a resonating capacitor (ballpark 22pF?) missing from the circuit across the 4 turn primary?
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1st Jul 2020, 11:00 pm | #10 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
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Re: 100MHz preamp
Is 9V enough to get cascode 2N3189s working well?
Colin |
1st Jul 2020, 11:17 pm | #11 | |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
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Re: 100MHz preamp
Quote:
I wonder if i connect a 10:1scope probe to the seconday of L2, would it load down the RF signal of L2? The coupling between primary and seconday is quite tight. i would use silvered plated ceramic trimmer or air variable cap for L1 centered at 100MHz and adjust the spacing for L1 to cover the full FM band Last edited by regenfreak; 1st Jul 2020 at 11:39 pm. |
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2nd Jul 2020, 12:58 am | #12 | |
Rest in Peace
Join Date: Jun 2006
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Re: 100MHz preamp
Quote:
However, having said all of that, I am in agreement with the posts above that refer to a low modulation index on BBC R3 FM. Al. |
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2nd Jul 2020, 2:31 pm | #13 |
Octode
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
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Re: 100MHz preamp
I take the comments about R3 being of low modulation index and that an increase in signal won't increase the audio output. I should have known that as its FM so increasing the signal wont increase the audio output.
Back to the amplifier. I have made a few changes as suggested by various people. I remade the primary tapping to a new position and added a trimmer to the output coil. With a single untuned turn as the output secondary connected to a scope there is amplification. Its difficult to quantify how much as the scope is rated to 50MHz and I'm running at 90MHz, so with the input at 2 divisions at 90MHz the output is at 4 divisions. This value is very dependant on the positioning of the output coil as you would expect. I have decided to leave the radio as it is and forget the preamp, I now know that it will work but won't make any difference. Thanks for the help in solving my problem. |
2nd Jul 2020, 2:42 pm | #14 | |
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Re: 100MHz preamp
Quote:
David
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2nd Jul 2020, 3:21 pm | #15 |
Octode
Join Date: Feb 2006
Location: Bristol, UK.
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Re: 100MHz preamp
There doesn't appear to be much background noise just low volume. With the volume control up high and no modulation(a Radio 3 pause) there's no hiss. So there is probably enough signal getting into the set it's just R3 having good dynamic range.
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2nd Jul 2020, 3:29 pm | #16 |
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Re: 100MHz preamp
So no need to change anything.
With a very low level FM signal, you will get an increase in volume as the signal is brought up, until the limiter starts to limit. Changes in volume then diminish to negligible, but instead the noise level is depressed. Once an FM receiver is into limiting, any interfering or co-channel signals are also suppressed. It's called the "Capture Effect" and it's one of the reasons aviation radio chose not to go to FM when everything else did. The other FM radio stations of similar signal strength will have the same dynamic range as radio 3 as far as the radio link goes, it's just the material being broadcast and the studio processing which chooses not to make any use of it. David
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2nd Jul 2020, 3:41 pm | #17 |
Octode
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
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Re: 100MHz preamp
Agreed. However it was a useful exercise in building something at RF which I haven't done for a long time. A lot remembered and learnt.
Thanks again. |