Oscillator questions. - Page 1
Forum section: electronics / general

author: Chris Wilson
author_rank: Super Contributor
timestamp: March 11, 2012, 11:17:38 am
content: General questions, but brought about by restoring a Yaesu FT-902DM early 1990's hybrid transceiver.I am very ew to electronics, please bear with my naieve questionsWhy do some receivers use three oscillators? This has a VFO operated by a tuning knob, a reduction gear drive, and a plate type capacitor. It also has a crystal controlled oscillator module, with crystals and associated electronics for each of the 12 amateur bands it utilises. Finally it has a voltage controlled oscillator module, with 12 varicap diodes for each of the twelve bands, tuned by a variable voltage input to the module, (from where?), with each band's varicap circuit switched on by a front panel rotary channel switch, by sending 8 volts to the required varicap circuit. The output of this module goes to the mixer and to the PLL mixer. I can't understand why three oscillators are needed, one being fixed, but switchable frequency, (the xtal controlled module) and two being variable frequency (VFO and VCO).Moving on. The xtal controlled module. The service manual states a range of voltage outputs that are deemed satisfactory, those being 80 to 150 mV Some bands were low, I *THINK*.However, looking at the output with my 150 MHz USB scope, I am unsure if this voltage is supposed to be peak to peak, or RMS? Each band section of the xtal module has its own xtal and transformer / capacitor pairing with a tunable slug. Now, turning the slug raises and lowers the output waveform *amplitude* until a point where the oscillations fade and then stop. According to my frequency counter the frequency never changes. My question is does this "tuning" just effect the amplitude of the output signal, and never affects the frequency? Can I just tune each transformer slug to give the desired output amplitude (and is it P to P or RMS....)? What effect does either amplitude extreme have, functionality wise?Moving to the VCO. Again, each of the 12 bands has its own varicap diode and a tuner inductor / capacitor circuit Now, am I right in thinki8ng these inductors *DO* and *SHOULD* effect the output frequency, assuming the voltage that controls the varicapcapacitance stays the same? I may be missing something basic here. I have a band (17 metres) where the VFO tuning knob stops altering the receiver's digital frequency display at the low extreme of the band, the display just stops decreasing for a few turns of the front panel tuning knob. Sometimes it then freezes.The tuning slug seems to shift the whole frequency span of the associated circuit up and down, until such a point oscillations stop. This seems a totally different function to the transformer slugs in the xtal oscillator module? How are these correctly set? My issue is the slug for 17 metres is cracked in half, at the bottom of the inductor, and being cracked locks up solid when turned by more than a fraction either way. Any tips to getting it out?How much commonality is there in these things, am I likely to find a none damaged replacement anywhere? There appears to be a mod on the VCO board that *LOOKS* factory, whereby instead of having separate outputs for PLL mixer and mixer, off two separate PCB connector strips, they are joined together, so both are effectively the same. The circuit diagram is athttp://www.chriswilson.tv/diagram.jpgSorry for the length of this, for every step I take I find I have a dozen questionsThe easy route is to find another complete VCO module, but I am using this transceiver repair to increase my knowledge of radio electronics.Thanks for reading.

author: Lawsen
author_rank: Frequent Contributor
timestamp: March 11, 2012, 09:31:37 pm
content: I have a Yaesu FT-817 and it is just a basic single VFO, variable frequency oscillator. Multiple oscillators are for different sets of frequencies in your radio and better tuning. I do not have a radio license to use my Yaesu FT-817. I just use it as a receiver as an music FM radio. I got it as a self gift. I find the user's control panel, not intuitive and the NiCd battery pack is not reliable and under rated for the load. The Yaesu FT-817 is not the best radio and transmitter out there. I never tried the transmitter.

author: siliconmix
author_rank: Frequent Contributor
timestamp: March 12, 2012, 03:14:46 pm
content: Quote from: Lawsen on March 11, 2012, 09:31:37 pmdont bother using it.it does a lot for what it is but even with impedance matching to antenna and it's ALC the final amp will go .

author: vk6zgo
author_rank: Super Contributor
timestamp: March 14, 2012, 06:29:45 am
content: Quote from: Chris Wilson on March 11, 2012, 11:17:38 amChris,the voltages will be RMS.Yes,the xtal oscillator coils are normally tuned for maximum output.This tuning should not normally affect the frequency unduly,as that is mainly determined by the crystal.That said,there is usually a fine adjustment of frequency provided to set the oscillator "spot on".Re the VCO:Yes,the inductorswillaffect the oscillator frequency.A so-called "VCO"is just an ordinary LC oscillator like the VFO,except that a part of the "C" is provided by a voltage controlled capacitor or "Varicap".The output from the VCO is (normally) digitally divided & compared to a crystal oscillator which has similarly been divided down.The comparison yields a DC control signal which is used to change the VCO frequency,in effect transferring the accuracy of the crystal osc to the VCO.(PLL circuit)The VCO frequency is tuned for different operating frequencies by changing the division ratio of the VCO divider.If you change the value of "L",the frequency will try to change,but will be "fought" by the PLL circuit,until ultimately the circuit loses lock.PLL circuits in Amateur Radio equipment are usually designed to reduce the output to zero in the case of a loss of PLL lock.(this may or may not be the case with other equipment).The twelve crystal oscillators are the references for the twelve VCOs.I hope I'm not freaking the EEV Bees out,by this,but I must point out that you are more likely to get useful info for this from QRZ.com,oreHam .com.as they are populated by Amateur Radio Operators many of have "hands on" experience with the FT902DM.Although many of the people on this forum possess formidable intellects,they may have to approach your question in a more general way than people who have actually done what you are attempting to do.73,VK6ZGOYeah guys,--"secret handshake"