[time-nuts] Oleg' s PN test Re: A new member & PN test set

[Bruce Griffiths]

You've missed the point which is that:
1) With 10MHz input frequencies the sum frequency is actually 20MHz which is beyond the first resonance of the inductor used.Something better is required. The sum frequency is the largest unwanted component that exits the mixer IF port.
2) Oleg is restricted in what he has available, rolling ones own conical inductor is an option given the iron powder Piconics claim to use is potentially available to him locally.

3) Oleg indicated he'd used the rig to measure PN with 60MHz inputs resulting in a 120MHz sum frequency output placing even more severe band reject requirements on the filter/diplexer.
4) The specified mixer is usable to 500 MHz with resulting sum frequency of 1GHz. If someone were tempted to use it at those frequencies as is the results would be "interesting" to say the least.
Obviously one could select off the shelf components that are satisfactory over a small frequency range or for a particular input frequency of interest.. This means the filter components have to be changed whenever the measurement frequency is changed too much.

However its probably more useful to use a filter that works well over the entire range of frequencies for which the mixer is useful.There are many ways to achieve this.

Thanks for the confirmation that home made conical inductors can work well. 
A version that worked from say 1MHz to 1GHz would be very useful.
Another potential problem is injection locking:If the VCXO/OCXO under test has inadequate reverse isolation then this can occur leading one to falsely conclude the VCO/OCXO under test has far better performance than it actually has. In effect the injected signal increases the PLL bandwidth far beyond the intended value.
Using high reverse isolation amplifiers on the mixer / phase detector input ports is one way of ensuring that injection locking doesn't occur.
Bruce
      From: Gerhard Hoffmann <dk4xp at arcor.de>
 To: time-nuts at febo.com 
 Sent: Thursday, 31 March 2016 12:12 PM
 Subject: Re: [time-nuts] Oleg' s PN test Re: A new member & PN test set
   
Am 30.03.2016 um 21:20 schrieb Bruce Griffiths:
> Conical inductors are available that are effectively resonance free to 40GHz
> but the largest value is around 10uH. In principle one could wind one's own
> conical inductor with a larger value, However an iron powder (carbonyl iron -
> available from Ukraine at least via ebay) and epoxy mixture. A cone angle of
> about 15 degrees appears to be suitable.
>
> Failing that, the classical method is to use a series string of inductors of
> increasing value. Even then the various resonances need to be damped.
> Lossy Ferrites and resistors can be useful, however one has to be careful not
> to increase the noise at frequencies of interest.

It doesn't take conical inductors to separate the baseband from the 
carrier at 10 MHz. The
world existed before Piconics and their conical L patents. Yes, we used 
them in our 10 GB/s
fiber optic transceivers, just to see what eye diagrams we could 
achieve. But at €38 a pop they
never ever made it into production. It was just too easy to replace them 
with somewhat more cent stuff.

A colleague even rolled his own from wire, epoxy glue and ferrite beads 
smashed in a mortar.
That looked, hmm, ugly, but performed excellently. Now, you get them 
from MCL and Coilcraft.

But for a 10 or 100 MHz lowpass, that's way over the top. Not even if 
you are nuts.

regards, Gerhard


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