Agreed.
Also, if there were so much loss as previously suggested, the inevitable
variation in relative phase error between switches would have to be so
bad that the unwanted sideband performance would be very poor indeed -
even at quite a low frequency, and this is not seen as being anything
like that bad.
As Bob says, you simply cannot ignore what is actually observed.
Bernard
G4HJW
Robert McGwier wrote:
> 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
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>
>
>
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