[time-nuts] Reverse isolation
bruce.griffiths at xtra.co.nz
Sun Mar 8 23:18:07 UTC 2009
NIST have measured reverse isolation as high as -144dB @ 10MHz for one
of their isolation amplifier designs but they gave no measurement system
Didier Juges wrote:
> As long as one is not trying to measure extremely low signals (or extremely
> high isolation), the conventional method works, and I have used it many
> times, I am not sure that the method would scale when dealing with very high
> levels of isolation.
> I certainly would not trust a conventional VNA for isolation of 100dB or
> more, simply because such isolation is difficult to achieve in an instrument
> that is supposed to be able to switch its source and receivers between the
> two ports.
> The HP 8722D which we use at work is only specified with 100dB dynamic
> range, and I am not sure how that applies to reverse isolation.
> Using a slightly offset test frequency while driving the device at its
> normal operating frequency would only work when using a spectrum analyzer as
> receiver. Most VNAs don't have the capability of rejecting large signals
> close in, and separating the frequencies too much would make the test
> invalid with a narrow band amplifier. I have used that method to measure the
> hot output VSWR of a TWT amplifier for instance (that was interesting). Of
> course, the spectrum analyzer does not give you the phase, but it's better
> than nothing.
> Didier KO4BB
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com
>> [mailto:time-nuts-bounces at febo.com] On Behalf Of John Day
>> Sent: Sunday, March 08, 2009 5:47 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Reverse isolation
>> At 06:14 PM 3/8/2009, Didier wrote:
>>> This question is directed at Bruce, but if anyone else has a
>>> contribution, feel free to speak.
>>> What is the best way to measure the reverse isolation of an
>>> (particularly a buffer amplifier for a 10 MHz reference), when it is
>>> expected to be in the order of 100 dB or more?
>>> Feeding the output with a known signal and measuring at the
>> input with
>>> a spectrum analyzer comes to mind, but I am sure there must be
>>> something wrong with that technique, it sounds too simple.
>> No, not really. How else would you measure it? Reverse
>> isolation is basically the reverse gain - S12 - of the
>> amplifier. How does a VNA measure S12? Essentially inject a
>> signal at port 2 and see how much comes out of port 1.
>> If you want to get so picky as to determine S12 with a signal
>> in the forward direction then you have a problem. Because the
>> forward gain -
>> S21 - is going to effectively swamp the signal going the other way.
>> So this is almost impossible to measure if the signals are at
>> the same frequency. In this case measure the S-parameters of
>> the amplifier in its 'normal ' configuration, then de-embed
>> the S-parameters of the device. Assuming the device is the
>> only non-linear element you are dealing with then from the
>> S12 & S22 values you can also figure out how the device
>> reacts in the reverse direction.
>> Then if you don't mind solving a large matrix you can figure
>> out how the device might react to passing a signal in both
>> directions. The reality is however that if the device is
>> within its linear range, which it is likely to be if you want
>> to distribute a reference or some such, the reverse behaviour
>> of the well terminated amplifier will approach the nominal
>> S12 value. The difficult part is to determine what happens
>> when the amplifier is not nicely terminated!
>>> The presence of a signal at the input (or not) may affect
>> the operating
>>> point of the amplifier, so measuring from output to input
>> without such
>>> signal may not give a true result.
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