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Old 20th Nov 2021, 9:45 pm   #1
Vintage_RC
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Default Thandar TP600 Pre-scaler

I recently picked up one of these as seen for less than a tenner on ebay. It seemed worth a punt as it could be used to extend the coverage of my TestLab VHF counter from its present 150MHz to 600MHz. On connecting up I was greeted with a reading of about 70MHz with the I/P grounded, this is indicative of the first stage oscillating at 700MHz. Further tests showed that as soon as a signal of a few tens of mV was present the reading was correct, ie it appears to be perfectly usable despite the no signal oscillation. The two main IC's are both Plessey, an SL952 UHF limiting amplifier and an SP8630B ECL divider, both IC's are dated 1983. I have attached a photo with the cover removed. Has anyone else got one of these? and if so is the no signal oscillation normal? At the moment I am minded to leave things as they are.
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Old 20th Nov 2021, 10:36 pm   #2
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Default Re: Thandar TP600 Pre-scaler

I have had a couple of 11c90 based prescalers and they did the same thing, about to find out if the MC12080 prescalers do it as well (when my new engraving bits finally arrive).

I would leave things as is for the moment until someone with the same unit can confirm if it is normal behaviour for that model.
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Old 20th Nov 2021, 11:10 pm   #3
chriswood1900
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Default Re: Thandar TP600 Pre-scaler

If you don't have the instructions here is a copy, There is a note in there that says with nothing connected to the input there can be a spurious output.
A similar circuit was used in the TF600 where it suggests that in the case of spurious readings check the decoupling capacitor/s on pin11.
I did not notice a problem with my TP600.
I hope that helps.
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Old 20th Nov 2021, 11:42 pm   #4
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Default Re: Thandar TP600 Pre-scaler

It's also worth digging up datasheets for those Plessey chips.

David
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Old 21st Nov 2021, 12:28 am   #5
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Default Re: Thandar TP600 Pre-scaler

I have a counter with a Plessey SP4740 prescaler that does the same thing.
Not sure grounding the input would do anything to stop it due to the capacitor.

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Old 21st Nov 2021, 12:40 am   #6
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Default Re: Thandar TP600 Pre-scaler

Thanks for the manual Chris and I note the comment in it about spurious O/P with no I/P. David (RW) I have managed to track down the data sheets for the Plessey IC's and the schematic in the manual seems to follow the (limited) application notes. David (factory) thanks for the additional snippet of information. From all this evidence it seems almost certain my unit is behaving in a typical manner.
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Old 21st Nov 2021, 11:40 pm   #7
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Default Re: Thandar TP600 Pre-scaler

My 1983 datasheet has under 'Operating Notes 2.'

'If no signal is present the device will self-oscillate. If this is undesireable it may be prevented by connecting a 15k resistor from the input to Vee (ie pin 10 to pin 7). This will reduce the input sensitivity by approximately 100mV.'

This paragraph was removed from later versions of the datasheet, maybe it didn't always work.

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Old 21st Nov 2021, 11:54 pm   #8
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Default Re: Thandar TP600 Pre-scaler

It didn't always work.

And it always set a threshold offset that seriously degraded sensitivity, usually out of spec.

Trying to make a very sensitive trigger for digital circuitry is not a particularly do-able proposition, the thing will always trigger on noise unless so much offset or (better still) hysteresis is employed that it significantly dominates all noise peaks. So much gain is involved that stability is not reliable.

For use in synthesiser dividers etc, reasonable operation is achieved through the certainty of a dominant signal. But even then, there is an effective lower frequency limitation or else a dv/dt limitation because the part will oscillate or give multiple noise spikes on threshold crossings.

There's a lot of subtle work in the input circuit design of a house-trained frequency counter.

Usually, basic instruments have poor sensitivity to cover the problem, but it still shows up under unfavourable circumstances.

THere's some coverage of these issues in the HP journal articles of various counters, but it was limited deliberately as commercially sensitive.

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Old 22nd Nov 2021, 12:15 am   #9
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Default Re: Thandar TP600 Pre-scaler

Does it oscillate with a resistive 50 ohm termination? If not then you could try fitting a resistive pi attenuator at the input. 3dB is 300R shunt, 18R series and 300R shunt so the minimum ESR is 18R. If it still hoots with the 3dB attenuator I suppose you could try a 6dB attenuator at the input. That would be closer to 50 ohms when mis-terminated at the input.

However, you could just leave it as it is if this instability isn't a problem for you.
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Old 22nd Nov 2021, 11:31 am   #10
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Default Re: Thandar TP600 Pre-scaler

Quote:
Originally Posted by G0HZU_JMR View Post
Does it oscillate with a resistive 50 ohm termination? If not then you could try fitting a resistive pi attenuator at the input. 3dB is 300R shunt, 18R series and 300R shunt so the minimum ESR is 18R. If it still hoots with the 3dB attenuator I suppose you could try a 6dB attenuator at the input. That would be closer to 50 ohms when mis-terminated at the input.

However, you could just leave it as it is if this instability isn't a problem for you.
When I referred to it oscillating with the I/P grounded, it was actually a 50 ohm BNC terminator on the I/P - so yes it does oscillate with a 50 ohm termination. I should have made that clearer in my original post.
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