[Elecraft] Re: MFJ259

[Jack Smith]

Yes, that's the basis of the N2PK VNA and it is worth looking at in a 
modified version, with I and Q detection.

The new DDS chips have simultaneous I & Q outputs, which makes that part 
of the design easier (N2PK design uses two AD9851's to generate 
quadrature output.) The newer chips are, however, on the small side.

Jack


Jim Miller wrote:
> is this an application where a direct conversion (QSD) might work better? 
> certainly easy to hit a narrow bandwidth.
>
> jim ab3cv
>
> ----- Original Message ----- 
> From: "Jack Smith" <Jack.Smith at cliftonlaboratories.com>
> To: <w3fpr at arrl.net>
> Cc: "Elecraft reflector" <elecraft at mailman.qth.net>
> Sent: Wednesday, March 14, 2007 6:14 PM
> Subject: Re: [Elecraft] Re: MFJ259
>
>
> Don:
>
> I have little or no problem putting an HP8752B VNA on my antenna
> feedline, or my N2PK VNA. Both have narrow band receive (adjustable down
> to 10 Hz in the 8752B) and hit the antenna with a reasonably hefty
> signal, 0 dBm or so. Consequently, unless you have a very strong signal
> inbound, measurements can be made without much trouble.
>
> The subject needs further though and library research, on the theory
> that an hour in the library can be worth a month in the lab.
>
> Jack
>
> Don Wilhelm wrote:
>   
>> Jack,
>>
>> Yes, connecting a measureing instrument directly to an antenna can be
>> troublesome if there are strong broadcast stations in the area or other
>> sources of signals that can fool the instrument.
>>
>> Zack Lau addressed that situation in a Technical Correspondence article in
>> QST a while ago - his answer was to measure the antenna indirectly with an 
>> L
>> network - the L network provides selectivity - adjust the L network to
>> provide a 50 ohm match, then remove it from the antenna, terminate what 
>> was
>> the measurement side with 50 ohms and measure the side previously 
>> connected
>> to the antenna.
>>
>> Sounds like a sane way to approach it to me.
>>
>> 73,
>> Don W3FPR
>>
>>
>>     
>>> -----Original Message-----
>>> From: Jack Smith [mailto:jack.smith at cliftonlaboratories.com]
>>> Sent: Wednesday, March 14, 2007 4:38 PM
>>> To: w3fpr at arrl.net
>>> Cc: Elecraft reflector
>>> Subject: Re: [Elecraft] Re: MFJ259
>>>
>>>
>>> After thinking on it more, 30 KHz bandwidth is too wide to be useful for
>>> a noise bridge detector, as it will not provide enough frequency
>>> resolution for the lower bands. 7-7.3 MHz would only have 10 discrete
>>> measurement points, for example.
>>>
>>> If the device were to be used only to measure components, and not
>>> connected to an antenna, lots of circuit simplification is possible in a
>>> bridge  generator / detector approach, i.e., a knock off of the classic
>>> GR916 RF bridge with an integrated source and detector. Like Boonton did
>>> with the 250A RX Meter, for example.
>>>
>>> However, as soon as you connect the unknown port to an antenna, simple
>>> detectors are out as the null will be submerged in trash unless the
>>> detector receiver has excellent selectivity and that means frequency
>>> stability as well.
>>>
>>> Jack
>>>
>>>
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