Re: [softrock40] VHF image cancelling DC recivers
- View SourceLawrence wrote:
> ----- Original Message -----I'll throw my two penneth in on this as well
> From: "Art"
>> But, I've been thinking quite a bit about this as I've
>> read the
>> various threads in the softrock group. And, I find myself
>> why the fascination with a 28 MHz transverter at all?
>> Sure, we can
>> get pretty sensitive transceivers for 28 MHz, so we
>> naturally use 28
>> MHz transverters since we already have the HF transceiver
>> in the shack anyway.
> There seem to be 2 reasons for the 28 MHz choice of IF.
> Firstly the width of the band on HF radios although that is
> not a problem for a narrow tuning range such as for
> moonbounce. Secondly the higher the IF the better the image
> rejection should be.
> A long time ago I used an IF range of 4 to 6 MHz for the
> European 2 MHz wide 2 m band. That converter was double
> conversion which would not be an advantage for this purpose.
> A relatively low IF, say around 10 MHz, could be OK with
> good filtering before the mixer. Alternatively an image
> cancelling mixer could be an option. To get the phase shift
> for the injection frequency we would probably have to use a
> tuned circuit, perhaps with a length of coax, rather than
> using digital means. I would also be inclined to use a
> couple of diode ring mixers rather than think of digital
> We need to have good image rejection both for dealing with
> unwanted signals and also to eliminate image noise in the
> system which is usually less obvious.
> 73, Lawrence GJ3RAX
I reckon I've spent most of the last 30 years with 144 MHz as my prime band.
I really do feel that all the hype about SDR systems is just that.
Use an SDR system as the IF after you have disposed of the huge problems
rejection , and you can have a really useful system.
What ever IF you choose you can guarantee that the commecial boys have
got some thing nasty on the image,
and some of the signals are very high, you can never be sure if the
interfering signals are intermod problems
form your own receive system or the crud is real crud.
You can get some very acceptable results just by pluging a softrock
straight after the first mixer, and before any filters, apart from the
post mixer diplex filtering.
Dump the synthesised LO as well, replace it with a crystal oscillator,
you can get much lower phase noise than a PLL or DDS LO.
Sample at a high enough rate to get the required rx band width and your away
As I said my two penneth.
73 Richard g8jvm