[time-nuts] Re: Noise down-converter project

[Lux, Jim]

On 5/16/22 8:11 AM, ghf at hoffmann-hochfrequenz.de wrote:
> Am 2022-05-16 15:16, schrieb Robert LaJeunesse:
>> FYI there are some rather flat video filter ICs that have been made in
>> the past. The 6th order HMC1023LP5E is tunable, at its 28MHz setting
>> its flat then down 0.1dB in the teens, down 0.35dB at 20 MHz. That
>> same setting is 60dB down at about 90 MHz. It is also a dual part,
>> designed for matched I-Q filtering.
>
> Declared dead at DigiKey.


Digikey is EOLing them - last time buy is July 31 2022Â  (in the US)


It's not entirely dead yet.  Mouser has them - they're marked EOL - but 
you can buy them for ~$40 each

This is one of those parts from Hittite (HMC partnumber) and they tend 
to do small runs, but on the other hand, if demand seems to pop up, they 
may make them again.

On the other hand, watch out for "custom parts" that just happen to have 
a Hittite part number.  At JPL, we had a vector modulator built by 
Hittite, it got a standard part number, and I assume you could buy them 
until they ran out. But I get emails every once in a while asking where 
to get that part we referenced.



>
>
>>> Sent: Sunday, May 15, 2022 at 5:29 PM
>>> From: "ed breya" <eb at telight.com>
>
>>> The actual filter I've been using does a good job on the higher
>>> frequencies, but is poor on flatness. It has about 2-3 dB p-p passband
>>> ripple, with periodicity around 5-7 MHz. I've tried various padding
>>> arrangements at both ends, all of which tend to flatten it only a 
>>> little
>>> bit at best. Looking at it with the TG/SA setup, the character is
>>> intrinsic to filter, and not due to just its reaction to the mixer and
>>> cabling and such.
>>>
>>> I hate building filters. Designing them in principle is easy, with all
>>> sorts of available tools online, but actually rounding up the real 
>>> parts
>>> (and their parasitics) and physical implementation is a PITA. But, I
>>> suppose I'll have to do it eventually for this project. I know how nice
>>> it can be, with the right filter, but for now, I'll have to go with 
>>> what
>>> I have.
>
> Did you choose a Chebyscheff design to start with? These accept some 
> ripple
> in the pass band, maybe some dB, to buy a steep rise of attenuation
> above f-3dB.


I agree with Ed here, easy in the tool, not necessarily easy in real 
life. One aspect of more "aggressive" designs - Chebyshev, Cauer, etc. 
is that they tend to be more sensitive to component variations - 
especially Cauer (Elliptical) because they depend on that carefully 
placed zero to get the rejection close to cutoff.