[hpsdr] 700MHz Full Power Bandwidth S/H

[Steve Bunch]

Maximo,

On Aug 29, 2010, at 11:01 AM, Maximo EA1DDO_HK1DX wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Hi,
> 
> Now I am beggining to understand how it really works...
> 
> I found that statement in internet;
> ...
> If the LTC2208 is able to sample up to 700 MHz so we could capture any 60 MHz bandwith between 1 and 700 MHz with the Mercury card, not just HF.
> Is that true?
> Is there any way to say to Mercury what 60 MHz to sample?

Yes, put a filter in front of it that excludes any signals other than the band of interest.  Given the good sensitivity of Mercury, it needs to be a pretty good filter.  If you do not filter out unwanted spectrum ahead of Mercury, any undesired signals that alias to the desired band will be inseparably present in the samples along with the desired one.  Do a Google search and look up the Nyquist zone concept.

However, as you go up in signal frequency, the signal-to-noise ratio you will achieve goes down by several dB per multiple of the sampling rate (Nyquist zone).  (Some A/D converter datasheets document their behavior for at least a couple of undersampling zones.)  By the time you get to 700 MHz, the quality of the signal you get by under-sampling will probably be worse than what you would get by using a mixer to move the frequency band of interest down closer to baseband.  You will still need to filter out unwanted signals to a very good degree if you use a mixer, but by careful choices of local oscillator frequency, you may move heavily-occupied areas of spectrum (e.g., the FM broadcast band) so it doesn't alias to the desired frequency range.  With pure under-sampling, the sampling frequency determines where the spectrum will be "folded over" onto the band you want, that is, where the Nyquist zones are.

Some people are using Mercury in the 2M band, and say they are getting good results.  You do need to bypass the internal low-pass filter, but that is straightforward (check the mail archives and wiki).  They are presumably using their Mercury's behind 2M bandpass filters and LNA's to cut down aliasing problems, and to achieve a good system noise figure.

73,
Steve, K9SRB

> Thank you
> 
> 73, Maximo - EA1DDO
> 
> 
> From: roland.etienne at free.fr
> To: ea1ddo at hotmail.com
> Subject: RE: [hpsdr] 700MHz Full Power Bandwidth S/H
> Date: Sun, 29 Aug 2010 15:58:31 +0200
> 
> Hi Maximo,
>  
> This means that it can be used with signals uo to 700 Mhz, but of course with a 130 Mbps maxi of sampling, you can expolit a 65 Mhz band pass up to 700 Mhz, for example 640 – 700 Mhz is possible.
>  
> 73, Roland f8chk.
>  
>  
> De : hpsdr-bounces at lists.openhpsdr.org [mailto:hpsdr-bounces at lists.openhpsdr.org] De la part de Maximo EA1DDO_HK1DX
> Envoyé : dimanche 29 août 2010 14:09
> À : hpsdr at openhpsdr.org
> Objet : [hpsdr] 700MHz Full Power Bandwidth S/H
>  
> Hi,
> 
> There is something that I do not understand. 
> If you see the LTC2208 specs at;
> 
> http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1155,C1001,C1150,P13693
> 
> There is a feature that I do not understand the meaning; 700MHz Full Power Bandwidth S/H
> 
> Any one is able to explaint it to me?
> 
> Thank you
> 
> 73, Maximo - EA1DDO
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