[DSP-10] an upgrade to the mini-circuits mar-6?

Wed, 12 Nov 2003 12:55:07 -0600

Hi John,

I admit that I have NOT used the MGA86576 in the DSP-10 (I haven't built it
yet!), but I have used the part in other places.  A couple of things you may
have missed, or didn't understand the implications of from the DEMI write-up
and data sheet:
1) Its not spec'd below 1.5 GHz. (Doesn't work all that well at lower
frequencies)
2) Its a HOT device (>22dB gain AT 4-6 GHz - will oscillate easily,
especially at low frequencies)
A MAR-6, on the other hand, has ~16 dB gain at 1 GHz.  For both parts, the
gain INCREASES as frequency DECREASES.  At 100 MHz (i.e., DSP-10), the MAR-6
is spec'd at 20dB. 

My experience says that the MGA86576 is much, much hotter.  It was unstable
at low frequencies.  Made a good oscillator at about 200 MHz.  (It was
supposed to be an amplifier, of course.)  I think it is likely the DSP-10
board would not work well with the MGA86576 due to the grounds not being
good enough, bypasses not good enough, etc.

The MGA86576 does work well at 1296 as a pre-amp, but with only a little
extra work, a GaAs-FET will work better.  (Biggest advantage seems to be
that it may be a bit more robust than a GaAs-FET preamp; i.e., it will
tolerate higher levels of input without damage.)  As Steve (DEMI) uses them,
they're great.  Inexpensive, repeatable, good NF and gain.  No tuning
needed.  Easier to get good results from that a GaAs-FET pre-amp.  But he's
also using them where they were designed to operate.

73 Donn
WA2VOI/0

>-----Original Message-----
>From: [email protected]
>[mailto:[email protected]]On Behalf Of John Scherer
>Sent: Wednesday, November 12, 2003 10:53 AM
>To: Dsp-10 (E-mail)
>Subject: [DSP-10] an upgrade to the mini-circuits mar-6?
>
>
>Hello All,
>
>I'm new to the group and haven't received my dsp-10 kit from & 
>DSP module
>TAPR yet.  I have been looking into mods for this radio, 
>specifically the
>MAR-6 amps and came across this:
>
>http://www.downeastmicrowave.com/upgrade.htm
>
>They use the HP/Avantek MGA86576 as a replacement for the 
>MAR-6 and claim a
>2-3 dB NF improvement!  Anyone ever try one of these?
>
>Here's the spec sheet: http://www.minikits.com.au/doc/mga86576.pdf
>
>Cheers
>
>John Scherer, KC0OIE
>
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