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Old 20th Dec 2022, 8:02 pm   #21
G6Tanuki
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

Surely the series-capacitor is just serving as a DC-blocker so the bias developed from rectified drive across the 6BW6's grid-resistor is not shorted out, and the coil is broadly parallel-resonant around 3.65MHz with the stray capacitance presented by the output of the second EF80 and the input of the 6BW6 ??
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Old 20th Dec 2022, 8:23 pm   #22
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

I have always thought of a coil and capacitor in series as a rejector circuit, in that it would appear as low impedance at it's resonant frequency, in which case if it was tuned at 3.5 MHz it would shunt output to ground, not double it, so wondering if the coil and cap is broadly tuned for 1.9 MHz, or have I got it wrong again?

A coil and cap in series (ie from signal source to signal load) is an acceptor cct at resonant freq. (ie, it allows that resonant freq. through), If applied across a point of signal down to earth, it is again an acceptor cct and shorts that resonant freq. down to ground (ideal for notch filtering).

Conversely, a parallel tuned cct wired in the signal path is a rejector cct and will act as a "trap" (Or notch if you prefer) of energy at resonant freq.
If wired across signal line down to earth it is once again a rejector and makes sure the resonant freq. signal is kept away from being "shorted" to earth on its path, so "boosts" only that resonant freq. if you like.

It is all down to usage.
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Old 20th Dec 2022, 8:32 pm   #23
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

I have used a coil and cap in series down to ground as a notch a few times which is why I tend to call them rejectors, so that is just my phrasing.

Looking at the circuit and seeing that C9 is switched in/out, C9 being 22 pF as it says that is the doubler, I am taking it as a coil and cap in series to ground. C8 which feeds the grid of V3 from the anode of V2 is 100 pF and R7 is 22K so that path is a fairly high impedance to ground.

As it does not give values for the inductance I am unsure as to what LC frequency is to be used. So I still see L3 C9 as a notch which is why I do not understand it?

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Old 20th Dec 2022, 8:39 pm   #24
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

The series tuned circuit is intended to reduce the level of the 1.75-1.9MHz fundamental fed to the output stage when used on 80m.

It should be noted that, while better than nothing, there is still a significant amount of the 1.75-1.9MHz present on the output of the AT5 when used on 80m. It really would benefit from a high pass filter to clean up the output spectrum.

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Old 20th Dec 2022, 8:51 pm   #25
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

Thanks Paula. So it is broadly tuned to 1.9 MHz and provides a low impedance at that frequency to ground so it would be better is it had a 3.5 tuned driver circuit in the anode circuit as well?

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Old 20th Dec 2022, 9:01 pm   #26
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

Here's a similar circuit from a mid-60s RSGB Handbook.

Text says "the second Z77 stage is designed to operate either as an untuned buffer amplifier on 1.8Mc/s or as a frequency doubler. The change-over is effected by switching in the inductance L2, in combination with the effective grid-to-earth capacitance of the PA valve this introduces a wide band tuned circuit between the driver valve V2 and the PA stage; the capacitor C13 serves merely as a blocking capacitor to prevent short-circuiting of the grid bias"

L2 is listed as "60 turns of 36SWG pile-wound on 3/8-inch diameter former with dust-iron core".

I guess you could work out what the likely inductance of L2 will be, and then see what frequency it would resonate at with a few tens of pF of stray-capacitance.
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Old 20th Dec 2022, 10:20 pm   #27
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

Well depending on how wide the coil is I get around 40 uH, if 20pF around 5.6 MHz so perhaps with the dust core it could go down to 3.5MHz?

I was looking at the picture pointed to in post #2 by paula and the coil in question looks virtually like half a choke around 1 mH?

I like the idea in the RSGB 1968 manual page 16.19 where it uses a tuned circuit in the buffer anode for 3.5 MHz and a choke for 1.8MHz.

Will have to play more.

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Old 20th Dec 2022, 10:47 pm   #28
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

C9 as wired with L3 in series are an acceptor circuit from the signal path to earth, so is a series notch filter.
I just looked up the cct diagram and C9 is a 22 pF so L3 must be a approx. 319 uH for an acceptor to stop about say 1.9MHz.
It wouldn't be anything else, that is the only purpose it could sensibly serve.

The RSGB cct I suspect is maybe different and is switching in the inductor as an RFC as it is wired in series with a very large cap that could be decoupling type value of 1nF or 1,000 pF. (or maybe not as it works in series with the series connected 330pF and 1 nF more likely on 2nd thoughts).
Too much thinking out loud, I must grab a beer
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Old 20th Dec 2022, 11:03 pm   #29
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

OK that echoes frsimen comments as being a 1.9MHz notch and my initial thoughts in post #20. It is just that being called a doubler for 3.5 MHz is throwing me off as I could not see it like that, hence asking the question.

In fact I can see no frequency doubling, just the notching of 1.9 MHz from driving the PA valve. I know the circuit must work or the AT5 would not have received good reviews, as I am trying to build my own variant of it and the many copies out there I would like to understand the circuit better in my own mind.

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Old 20th Dec 2022, 11:10 pm   #30
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

This is from the 1968 RSGB manual and has the broadband for 1.8 MHz with a choke and what I would recognise as a doubler for 3.5 MHz.

If I can I will try for this type of circuit.
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Old 20th Dec 2022, 11:11 pm   #31
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

In fact I can see no frequency doubling, just the notching of 1.9 MHz from driving the PA valve. I know the circuit must work or the AT5 would not have received good reviews, as I am trying to build my own variant of it and the many copies out there I would like to understand the circuit better in my own mind.

I personally would have two switched parallel resonant ccts desiged for 1.9 and 3.5 MHz resonant ccts in the anode of the driver valve L2 which I notice is a currently 2.5 mH wideband choke.
Maybe one coil and a capactor switched in or out could achieve this aim, or a tapped and switched coil if not.
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Old 20th Dec 2022, 11:13 pm   #32
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

This is from the 1968 RSGB manual and has the broadband for 1.8 MHz with a choke and what I would recognise as a doubler for 3.5 MHz.

If I can I will try for this type of circuit.


"snap" That looks better to my eyes
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Old 20th Dec 2022, 11:17 pm   #33
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

The measured value of L3 is 195uH. It looks like the designer was trying to pull off having both a peak response at 80m and a rejection response at in the 160m region. LTSpice shows that the peak response is fairly easily achieved, the rejection frequency is rather high with a 22pF capacitor. The end result depends a lot on the stray capacitance.

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Old 20th Dec 2022, 11:26 pm   #34
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

195 uH and 22pF would give a resonant frq. of 2.3 MHz approx. according to me after a few beers and an old calculator with sticking buttons lol.
Assuming that 195 uH is correct...
Was there an bad response at 2.3 MHz they tried to tame?
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Old 20th Dec 2022, 11:33 pm   #35
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

frsimen I have nevwe used LTspice, perhaps something I may look at some time, can you please tell me what components is causing the peak response? as I still only see a notch?


I have also decided to go for a different bottle then the 6BW6, I am using a 6V6G, which is said to be the same valve characteristics, but with a larger glass may be a bit cooler.

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Old 21st Dec 2022, 12:25 am   #36
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

I measured the inductance of L3 using my Wayne Kerr meter which only tests at 1kHz. It's possible that the value of L3, with its ferrite core, read lower than its true value.

If you factor in the grid circuit capacitance of the output valve and the output capacitance of V2, they appear in parallel with the L3/22pF series combination. Add in a little self capacitance for L3 and you end up with a notch response due to the L3/22pF and a higher frequency resonance due to L3 and the various strays in series with the 22pF.

I've not noticed any problems with the AT5 that would need taming. As it is a simple VFO/multiplier design, there isn't much opportunity for non harmonically related issues. The multiplier stage is stable when used on 160m, and it doesn't seem to misbehave when used on 80m.

I hope your AT5 clones don't drift as much as my real one does!

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Old 21st Dec 2022, 1:56 am   #37
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

Quote:
Originally Posted by frsimen View Post
I hope your AT5 clones don't drift as much as my real one does!

Paula
Is that thermal drift? C1 and L1 will probably have large positive tempco. Looking at the parts list C3 & 4 are just listed as 270pF. If those are not already polystyrene caps (i.e. negative tempco), then making them so might improve thermal stability. C5 is listed as 10-40pF "temp.comp". Is that one of the Oxley temperature trimmers? Wonder what it is if not?

B
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Old 21st Dec 2022, 4:05 am   #38
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

Ah, but there is the anode-to-ht inductor of that same second EF80, so you get a tapped tank, not a simple series one. Two inductors are in play.

The circuit has both a series mode resonance and a parallel mode resonance.

Alternatively, if that series mode capacitor is big enough to be a decoupler, you could see the new inductor introduced when the switch is closed as being simply jammed across the EF80 anode inductor.


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Old 21st Dec 2022, 12:10 pm   #39
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

A shot of the inductors L2 & L3 associated with V2 (EF80) can be seen here starting at 8:03 in :

https://www.youtube.com/watch?v=G_oce-PsOcM

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Old 21st Dec 2022, 1:24 pm   #40
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Default Re: Quick question on the CODAR AT5, if anyone has one to hand.

Pragmatically, I have never liked feeding the grid of an output-stage direct from a doubler (or indeed any stage that may be operating in a mode which _could_ constitute a doubler - such as a Class C driver) without some form of signal-frequency filtering [traditional L:C or a bandpass coupler] in between the driver stage and the PA grid.

In that respect the second RSGB circuit makes more sense but even on 1.8MHz I would prefer to see a tuned-circuit as the anode-load rather than a choke.

A pi-tank output tuned to 3.5MHz does very litttle to stop inadvertent 1.8MHz energy going up the antenna!
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