[Elecraft] Re: MFJ259

[Jack Smith]

Don:

My thought was a single conversion receiver with a high IF (45 MHz) with 
a monolythic crystal filter, and an AD8307 as the IF amp and detector.  
Use an NE602 or 612 oscillator/mixer. Throw in a 30 MHz LP filter to 
knock down images. Local oscillator runs 45-75 MHz.

These filters are about 30 KHz wide, and not very expensive. Their 
impedance is a good match for an AD8307 and the NE602.Use a PIC as a 
frequency meter, maybe a huff-n-puff stabilization arrangement with the 
PIC as  well, or partial stabilization as used in the HP8640B signal  
generator's PLL circuit.

If the objective is to come in at the $100 level or less, this approach 
might make it. There's not a lot of budget for parts with that target price.

Jack



Don Wilhelm wrote:
> Jack,
>
> How about putting a sharp tuned circuit between the detector output from the
> bridge and the AD8307 - a single parallel tuned circuit with a variable cap
> and a winding on a toroid can serve with high Q over a wide frequency range
> (depending on the variable capacitor size) - not much different than tuning
> the receiver.  An input link on the toroid can match the 50 output of the
> bridge and a third link can match the input impedance of the AD8307.
>
> Of course the detector need not be so exotic if a narrow band signal source
> is used (like a QRPp transmitter), and that can simplify not only the
> bridge, but the detector too since more power is available.
>
> Ideas and alternatives abound - all with tradeoffs for accuracy, ease of
> construction, ease of use.
>
> Yes, the number of knobs grows, and likely limits use at the top of a tower,
> but half wavelengths (or multiples) of coax are handy in that situtation and
> allow the measurements to be made on the ground - ideally by a ground crew
> member and the antenna tuning done by the person at the tower top.
>
> 73,
> Don W3FPR
>
>   
>> -----Original Message-----
>>     
>
>   
>> Don:
>>
>> I've been  giving the subject a bit of thought as well and a noise
>> bridge came to mind rather quickly. I have an old MFJ 202 noise bridge I
>> bought 25 years ago.
>>
>> However, I believe a broad band detector such as the AD8307 will fail.
>> The null condition is rather narrow frequency wise, and the noise
>> generator detector output is broad, so the different in total power at n
>> AD8307 detector between no null and perfect null will be tiny,
>> particularly if the bridge is constructed to have good balance, which
>> is, of course, the objective.
>>
>> This is a case where one needs a narrow band tuned detector, I believe.
>> Or, a narrow band source.
>>
>> Jack K8ZOA
>> www.cliftonlaboratories.com
>>
>>
>> Don Wilhelm wrote:
>>     
>>> Those who have both an Elecraft N-Gen and a KX1 have everything needed
>>> except the bridge elements to construct and use a relatively
>>>       
>> compact version
>>     
>>> of that Improved Noise Bridge described by John Grebenkemper in
>>>       
>> that August
>>     
>>> 1989 QST article.
>>>
>>> The N-gen is the noise source and the KX1 can serve as the
>>>       
>> detector - listen
>>     
>>> for the null.  Of course, it is easy to construct the noise
>>>       
>> generator, so
>>     
>>> those with only a portable receiver like the KX1 can have an accurate
>>> impedance meter once it is calibrated.
>>>
>>> Detectors other than a receiver can also be used - an AD8307 based power
>>> meter should work just fine and fit into a small case.
>>>
>>> The 'disadvantage' of the noise bridge is there are two knobs
>>>       
>> to deal with,
>>     
>>> but when a deep null is achieved one can read both the resistive and the
>>> reactive component of the unknown impedance.
>>>
>>> 73,
>>> Don W3FPR
>>>
>>>
>>>       
>>>> -----Original Message-----
>>>>
>>>>         
>>>       
>>>> Dave G3YMC wrote:
>>>>
>>>>
>>>>         
>>>>> I would also make a plug for a noise bridge. I built the one by KI6WX
>>>>> which appeared in QST in August 1989 (available to download from the
>>>>> ARRL site). If you have a suitable portable rig to couple to it does
>>>>> most of what the fancy analysers can do at a fraction of the cost.
>>>>>
>>>>>           
>>>> It should not be too difficult to design a fairly simple receiver
>>>> drawing a
>>>> small amount of current to be enclosed with a noise bridge to
>>>> make an 'easy
>>>> to carry' instrument. Top of head thinking.
>>>>
>>>> 73,
>>>> Geoff
>>>> GM4ESD
>>>>
>>>>
>>>>
>>>>         
>>> --
>>> No virus found in this outgoing message.
>>> Checked by AVG Free Edition.
>>> Version: 7.5.446 / Virus Database: 268.18.11/721 - Release
>>>       
>> Date: 3/13/2007
>>     
>>> 4:51 PM
>>>
>>> _______________________________________________
>>> Elecraft mailing list
>>> Post to: Elecraft at mailman.qth.net
>>> You must be a subscriber to post to the list.
>>> Subscriber Info (Addr. Change, sub, unsub etc.):
>>>  http://mailman.qth.net/mailman/listinfo/elecraft
>>>
>>> Help: http://mailman.qth.net/subscribers.htm
>>> Elecraft web page: http://www.elecraft.com
>>>
>>>
>>>       
>> _______________________________________________
>> Elecraft mailing list
>> Post to: Elecraft at mailman.qth.net
>> You must be a subscriber to post to the list.
>> Subscriber Info (Addr. Change, sub, unsub etc.):
>>  http://mailman.qth.net/mailman/listinfo/elecraft
>>
>> Help: http://mailman.qth.net/subscribers.htm
>> Elecraft web page: http://www.elecraft.com
>> --
>> No virus found in this incoming message.
>> Checked by AVG Free Edition.
>> Version: 7.5.446 / Virus Database: 268.18.11/721 - Release Date:
>> 3/13/2007 4:51 PM
>>
>>     
> --
> No virus found in this outgoing message.
> Checked by AVG Free Edition.
> Version: 7.5.446 / Virus Database: 268.18.11/721 - Release Date: 3/13/2007
> 4:51 PM
>
>
>