EMRFD Message Archive 185

Message	Date	From	Subject
185	2006-11-07 10:44:15	Nick Kennedy	Up-conversion
If one were designing a (conventional; analog) receiver or transceiver for HF, what are the advantages of up-converting to an IF around 70MHz? I've been re-reading Ulrich Rohde's QST articles and see that scheme in his analysis of a TS-50 and in his own proposed design. I guess it's common these days. I can see that having the LO tune one continuous range, always in the same direction, might make you want the IF to be out of range of the desired band. I ask because of the necessity of having a narrow (as possible), steep sided filter following the IF. For a ham of modest means and test equipment, a 9 MHz filter would be much easier to design than a 70 MHz one. So are there reasons other than what I stated for preferring the IF so high? Before I even push the 'send' button, I suppose that image rejection is going to be a big one. What else? TNX/73- Nick, WA5BDU
186	2006-11-07 12:14:42	larry allen	Re: Up-conversion
Actually, having an upconverter as you first IF is a good idea...This way you can also have your receiver receive frequencies like 2 meter and 450.. The first IF is high enough to descourage images.... Second, I would not worry about needing a narrow filter at 70 mhz... as most receiver designs put their filter money into the lowest IF... eg digital IF's tend to be as low a frequency as practically possible.. eg IF's of 70 mhz... mixed down to 9 mhz .. mixed down to 2 mhz.. mixed down to 455.. mixed down to 100 khz... With technology as refined as it is mixer noise is very low.... Larry ve3fxq ----- Original Message -----
188	2006-11-07 16:30:35	jr_dakota	Re: Up-conversion
Image rejection and eased front end filtering requirements ... also it makes continuous LF-HF recepti
199	2006-11-12 13:54:57	Luiz Amaral	Re: Up-conversion
It is a great advantage to use an IF above the maximum receiving frequency because it is possible to get a ?all HF? radio without the problems: 1 - reception of any frequency (if you have a 9 MHz IF, for example, signals of 9 MHz cannot be received without zero leakage to the IF circuitry). 2 - The image rejection becomes much better because they are separated from the desired signals 2 x IF, or 140 MHz. Luiz - PY1LL '>'-- Mensagem Original -- '>'To: emrfd@yahoogroups.com '>'