Re: LF: RTE 252kHz

[Andy Talbot <andy.g4jnt@gmail.com>]

Hi All - 

Its no good at this stage looking for small signals.  All I'm trying to do is prove the HARDWARE.  At the moment I have a QSD type receiver delivering a signal centred on 1kHz.    This is bandpass filtered to about 100Hz bandwidth, then I/Q sampled at 1kHz sampling rate to deliver 12 bit baseband values at 1kHz

At the moment all I'm doing with these is  displaying them on a vectorscope plot - like that shown in the attachment - in order to prove phase coherency of the system.  So only strong signals are acceptable at this stage - decimation and narrowband filtering can come later - that is high level software and easier to implement

And I've discovered something that needs explaining.  Having just been monitoring Droitwich on 198kHz for the last couple of hours, its phase plot indicates drift of 300 degrees in that time.   Expressed in terms of a frequency offset that is 0.00012Hz, or 0.6 PPB at 198kHz.

Now :

My reference is a VE2ZAZ GPSDO which is long term spot on, but subject to short term variations.   I have to check if it remains better than 0.6PPB over that period.    The VE2ZAZ isn't my favourite solution for LF working.  But a local high standard frequency reference isn't an issue - the Caesium tube can be turned on to solve that one!

The LO in the LF receiver is a DDS tuned by a rotary encoder.    The PIC maths was written from first principles using 64 x 64 bit integer arithmetic, BUT the pre-stored -constants to start with were evaluated via a spreadsheet - which I'm not convinced provides sufficient numerical accuracy to simulate the maths resolution needed to get teh constantsin teh first place.   So that needs checking.

And, of course,   although Droitwich is supposed to be a frequency standard, it is only Rubidium controlled; set manually from "time to time" against an on site Caesium source.  0.6 PPB is of the order of a Rb source that hasn't been corrected for a while.   Is Droitwich properly maintained these days when very few people want it as a reference?  can we really assume its good enough?

I've now changed to monitoring MSF 60kHz so lets see what indicted drift that shows - its reasonable to assume that really is good!

Checking the constants used in the arithmetic is the most difficult task.  My high level programming language only offer up to 64 bit integers, so the multiplication used in the PIC cannot be simulated exactly in a prog, meaning I need to work with double precision floats and cast to integers as and when.

Andy  G4JNT

On 1 November 2015 at 18:13, IZ7SLZ <iz7slz.domenico@gmail.com> wrote:

On 11/1/2015 4:17 PM, Andy Talbot wrote:

Oh well...

Still searching for a clean MF carrier

Andy   G4JNt

Hello Andy, and all

i'm now putting a small signal on 478502.24 Hz in opera8 mode. Tx is GPS-locked. It is the same TX exciter that i used in EbNaut experiment  weeks ago.
Maybe this signal is useful for your receiver test.

All the best.

73, Domenico / iz7slz