[Lowfer] R75 Frequency Accuracy
pbunn at patbunn.com
pbunn at patbunn.com
Wed Jan 23 21:37:52 EST 2013
I have let mine go for the last 6 hours from cold start to relatively stable
and it is 15Hz low and stable - I am transmitting a -55dB signal to my
antenna at 502.00 KHz signal from my 8656b that is GPS disiplined. It reads
502000 Hz on the GPS disiplined HP Counter. The receiver puts it on 700 Hz
on ARGO when the RX dial read 502015. I have calibrated ARGO with a GPS
disiplined HP 3225A at 1000Hz to about .05Hz.
I will adjust L2 on the PLO board and see how close that I can get it.
Probably end up using Jays circuit and trying to disipline it but first I
want see how close that I can get it.
BTW - The Thunderbolt is really nice and it makes the old HP stuff really
useful.
Thanks everyone for the assistance.
Pat
N4LTA
-----Original Message-----
From: Douglas D. Williams
Sent: Thursday, January 24, 2013 2:09 AM
To: Discussion of the Lowfer (US, European, &,UK) and MedFer bands
Subject: Re: [Lowfer] R75 Frequency Accuracy
The main shortcomings of the R-75 for direct VLF/LF receive occur
mostly below 200 kHz, where there are numerous spurs that can be heard with
nothing but a shielded 50 ohm "dummy load" connected to the antenna input
jack, and as noted in two separate Lowdown reviews of two different R-75
receivers. The further below 200 kHz you go, the more numerous the spurs.
The R-75 can only directly tune down to 30 kHz. A good quality upconverter
eliminates almost all of the spurs, and allows the reception of down to
around 5 kHz, but also introduces additional frequency drift.
I agree that the R-75 is very stable, especially with the TCXO option.
73, Doug KB4OER
On Wed, Jan 23, 2013 at 8:34 PM, <jrusgrove at comcast.net> wrote:
> Same here. Have two R-75s that are used almost daily for LF/MF activities.
> Both have the TCXO option and remain within a tenth of the indicated
> frequency. Agree with John ... the shortcomings of the R-75 have been
> overstated. Both are used to receive LF and MF directly without
> upconversion. Performance in this application is on par with the the
> Harris
> RF-590 - the main LF/MF receiver - and other 'premium' receivers in use
> here.
>
> Jay W1VD WD2XNS WE2XGR/2
>
>
> ----- Original Message ----- From: "John Andrews" <w1tag at charter.net>
> To: "Discussion of the Lowfer (US, European, &UK) and MedFer bands" <
> lowfer at mailman.qth.net>
> Sent: Wednesday, January 23, 2013 7:59 PM
> Subject: Re: [Lowfer] R75 Frequency Accuracy
>
>
>
> Pat,
>>
>> Like Dex, I keep the R75 on all the time, so the warm-up period is not a
>> factor. I don't use an upconverter, so I am receiving directly through
>> the
>> R75. The R75's shortcomings have been overstated, but I really don't want
>> to get into an argument over it. It has worked out very well for me in
>> all
>> applications.
>>
>> Regarding calibration, it would be worth the time to take the top cover
>> off, and adjust L2 in the PLL unit. I would recommend doing that up at 10
>> MHz, since you now have a good source. You can do this with Argo,
>> assuming
>> that you have done the sound card calibration first so that the only
>> error
>> would be in receiver.
>>
>> At LF, my R75s are both within a couple of tenths of a Hz of being on
>> frequency.
>>
>> John, W1TAG
>>
>> On 1/23/2013 6:37 PM, pbunn at patbunn.com wrote:
>>
>>>
>>> I have an older R75 that I bought 15 years ago and rarely used until
>>> recently.
>>>
>>> I use it for LF work with a home brew RX up-converter and it has worked
>>> well but frequency accuracy has been quite a bit off. I attributed this
>>> to my up converter being a bit off and combined with the R75 error
>>> combining to cause the problem.
>>>
>>> Recently I have added a Trimble Thunderbolt and have begun to find out
>>> where the error is. My up-converter is now phase locked to the GPS 10
>>> Mhz output and yet my receiver is still 8 Hz off .
>>>
>>> Is this more than might be expected from a typical R75 or normal. It
>>> seems somewhat excessive to me. I plan to add W1VDs circuit to add GPS
>>> control to the R75 but want to make sure that the R75 is not excessively
>>> out of spec.
>>>
>>> The sound card was calibrated with a GPS disiplined 1KHz signal and
>>> found to be within .1Hz using ARGO before calibration so the error is
>>> not there.
>>>
>>> Any comments would be appreciated.
>>>
>>> Pat
>>> N4LTA
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