EMRFD Message Archive 4038

Message Date From Subject
4038 2010-01-19 17:53:47 john Mixer from TV tuner
Hi Folks....I picked up a dead VCR player from a neighbor, tore it apart and saw what was expected, many SMT parts....I pulled the mixer from the TV tuner that is packaged in a can like a TUF-1 mixer with manufacturing part numbers from Japan on it that can not be Googled.

I'm wondering if anyone might have a good idea what the Low and High Frequencies might be of the mixer as well as the LO injection level, etc. Many thanks, John K5IRK
4041 2010-01-19 21:30:38 KK7B Re: Mixer from TV tuner
Hi John,

If it's in a TUF-1 type can with 4 leads, it's pretty easy to measure. One lead is probably connected to the can, and you may be able to see the connection. Assume a standard diode ring mixer schematic. I think you've build those from scratch before.

The LO and RF leads are probably connected to transformer windings, with the other end of the winding connected to ground. You can confirm that with an ohm meter. LO and RF ports are sort of interchangeable--but usually you put the LO drive into the isolated pin on the end past the ground connection.

The IF lead is often connected directly to the diodes.

With a few volts and a series current-limiting resistor, put a few mA DC into the IF port to turn on one pair of diodes.

Now you have a test circuit consisting of two back-to-back transformers with a pair of turned-on series diodes in between.

Connect some coax to the RF and LO ports, and sweep it with a 50 ohm power meter and signal generator--that will determine the useful frequency range of the RF and LO ports. The IF port will probably extend from DC up to that frequency. Since you've turned on the diodes with DC, keep the RF signal level below about 0 dBm for the loss measurement. You are mostly measuring the transformer loss across the frequency range. That will give you the useful frequency range of the mixer.

Standard LO injection level is +7 dBm. Try it there, and then raise the LO injecti
4042 2010-01-20 04:58:11 john lawson Re: Mixer from TV tuner
Good morning Rick and Harold. many thanks for the suggestions. Harold I assumed it was a mixer, but obviously I can check that out using Rick's guidance on the mixer which I found very helpful and instructive.
Rick, yes it does have the 4 pins like a TUF-1 and the grounded pin is visible as I kept the mixer (?) attached to the PC board chunk that it was mounted to.  Thanks again to both of you....Have a good day, 73 John K5IRK