"Bob Henderson" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- I have now used my R-4C for a few days after having adjusted the 1st mixer injection ratio to around 9:1. What I have found has been interesting. The third order IMD response in practice appears vastly better than the 70dB I measured after I'd increased the injection ratio.
Prior to the change, I was suffering from heavy BC generated IMD products on 40m. In particular at dusk, end-stopping signals on 7115 and 7165 combined to produce an S9+ heavily distorted IMD product on 7065. With the increased injection ratio, I now find that the end stopping signals on 7115 & 7165 produce nothing detectable on 7065 even with an S1 noise level. This implies a wide spaced IMD3 response of around 100 dB. That seems pretty unlikely but it certainly confirms what I have to be better than the 70 dB I had measured. Back in the workshop, I set about re-testing for IMD3 response. What I found was that my previous test had been flawed due to a change in input levels. This time my measurements suggested a figure of 90 dB or better. More like it ! The main difficulty I have had in making progress with this problem has been the Heath Robinson nature of my test set-up. The measurement inconsistencies I have suffered have sent me on several wild goose chases. Consequently, I have vowed to construct a more satisfactory test set-up. What I currently have in mind, is to build two seperate low noise xtal oscillator & buffer amplifier circuits producing a 20 dBm output. One oscillator fixed on 7040 and the other switchable 7039.5, 7039, 7038, 7035, 7030, 7020. I have chosen this frequency as 7040 xtals are available cheaply ($2.50) due to their extensive use by the QRP fraternity. As yet, I am undecided whether to use several xtals in oscillator 2 or just make it a VXO. Either way, I expect the 7040 xtals can be pulled adequately to provide most of the required frequencies. I haven't settled on the devices to use in the oscillator but believe that low noise CATV devices will probably be a good choice. I have managed to obtain some appropriate step attenuators quite cheaply, so I plan to run each oscillator output through its own attenuator chain then through a combiner. This arrangement will allow both signal outputs to be adjustable in 1 dB increments from +20 dBm thru -150 dBm and provide a source to source isolation at -13 dBm (S9+60) of better than 90 dB, which will hopefully ensure the output remains essentially intermod free. If anyone has already done something like this or feels a different approach might be better, I would appreciate your advice and being able to share further thoughts with you. 73 Bob, 5B4AGN ---------------------------------------------------------------------- On Behalf of "Bob Henderson" <[EMAIL PROTECTED]> Submissions: drakelist@www.zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: http://www.zerobeat.net Brought to you courtesy of TLCHost.net http://www.tlchost.net/ ----------------------------------------------------------------------
