Art:
Let me reiterate what it is we OBSERVE and can measure with the QSD in
the SDR-1000.
We do /***NOT***/ see conversion loss increase as the frequency gets
higher. We see a nearly flat 1 dB insertion loss (to withing reason but
so close, just fogettaboutit).
What we do see is a steady rise it the noise floor. If you put in a -70
dBm signal at 14 MHz, you got approximately the same converted voltage
as you do at 54 MHz with -70 dBm signal. The MDS gets worse, the
loss does NOT.
Bob
N4HY
KY1K wrote:
> Below is a copy of a message I posted to the softrock-40 group. Some
> who monitor here might be interested as well.
>
> Message follows:
>
>
> -------------------------------------------------------------
>
> Greetings all,
>
> I simulated (Spice) the sluggish turn off times of the FST3125 cmos
> switch. Due to the slower turn off time, there is overlap in the
> conduction of adjacent switches...meaning that 2 switches are
> momentarily conducting at the same time. I found the conversion gain
> drops very quickly once the 25 percent duty cycle is exceeded and
> that this condition should be avoided for good results.
>
> I had hoped to release the data in a more polished form, but here is
> the summary of the losses for each HF band.
>
> These are relative conversion gains and were obtained by applying 4
> nS overalp on each analog switch. Note that ~4 ns overlap is the
> actual overlap that exists in the FST3125 and similar technology cmos
> analog switches we use in softrock's/QSD's today.
>
> On 50* Mhz, the conversion gain is down 20.3 db (relative to a
> perfect 25-25-25-25 duty cycle).
>
> On 28* Mhz, the conversion gain is down 14.2 db (relative to a
> perfect 25-25-25-25 duty cycle).
>
> On 14* Mhz, the conversion gain is down 8.7 db (relative to a perfect
> 25-25-25-25 duty cycle).
>
> On 7* Mhz, the conversion gain is down 7.6 db (relative to a perfect
> 25-25-25-25 duty cycle).
>
> On 3.5* Mhz, the conversion gain is down 4.6 db (relative to a
> perfect 25-25-25-25 duty cycle).
>
> *4 ohm R(on), a 10 Megohm R(off), with 4 nS overlap, .01 uF integrating caps.
>
> I also have the Spice model(s) used, and a graph showing relative
> conversion gain vs the switch duty cycle for ranges between 5 and 45
> percent. The graph is a crude gif file, but is legible. Email me OFF
> LIST if you would like any of the data.
>
> Regards,
>
> Art
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>
>
>
--
AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
Laziness is the number one inspiration for ingenuity. Guilty as charged!
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