I have another preamp that uses an old MRF-904. Have you characterized that
transistor at 1.8MHz? Is it worth replacing with a 2N5109?
Art
ᐧ
On Tue, Sep 22, 2015 at 3:16 PM, Richard (Rick) Karlquist <
richard@karlquist.com> wrote:
> On 9/22/2015 11:40 AM, Art Snapper wrote:
>
> The KD9SV pHEMT FET preamp is a nice addition to the shack. I was able to
>> do an in/out test this morning, and it made a big improvement. This was
>> the
>> first time since installation during the summer, that the atmospheric
>> noise
>> was low enough to give it a decent test.
>>
>> de Art NK8X
>> S.W. Michigan
>>
>> Just an FYI:
>
> After seeing this posting, I looked at various KD9SV products.
> The pHEMT preamp is advertised as "Featuring components with rated noise
> figures as low as 0.4 dB". Yes, but pHEMT's are not specified
> at 1.8 MHz, and for good reason. They have a boatload of 1/f noise.
> The NF is many dB higher at 1.8 MHz (how many dB? well it's not
> specified so you have to test each one.) But it is bad. Other
> KD9SV use MOSFET's like BF981. These also have high 1/f noise
> at 1.8 MHz. These 1/f issues are well known in the industry,
> although maybe not to the designer. It isn't clear that the
> NF was actually measured.
>
> I spent a lot of time characterizing pHEMT's and MOSFET's in
> hopes of making a low noise preamp for 160 meters. They are
> non-starters, IMHO. I specifically tested the BF981.
> You could do much better with bipolars like 2N5109 or JFET's.
>
> Rick N6RK
>
>
>
>
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|