Hello experts…i am new in the RF field and i need some help…
So i have a double balanced mixer (ZAD-6+ of Mini Circuits) that need +7dBm Local OsC power and 30MHz Loca.osc frequency. At the time i use signal generator for l.o
How can i make a crystal oscillator with these parameters? Can anyone tell me simple how to start? I can buy also some ready crystal osc ?
Many thanks
George
What you need to build is a XTAL oscillator followed by an RF buffer then a low pass filter and an RF amp to supply the +7dBm power to the mixer. The circuit may need a 50 Ohm attenuator pad between the RF amp output and the mixer to ensure a 50 Ohm impedance match. The easiest RF amps to use are the ‘mimic’ IC amps as they have 50 Ohm input and output matching.
This may not be the best forum for this question. I suggest searching for Amateur Radio (Ham Radio) web sites, books and forums. This is a common circuit used in Ham radio equipment building and there is much info on these.
You can buy “canned” 30 MHz crystal oscillators from Digikey or others, but they will have a square wave output that must be well filtered, to produce a pure sine wave, before feeding into the mixer.
Diode mixers work best with square wave inputs.
Hi friends and really thanks for your replies. So…
@waltr: the RF-Buffer to match the oscillators output impedance with the filter’s input impedance right? You know you thoughts is really good but i don’t know i)about the frequency stability that i need to feed the mixer.All these devices i am afraid will slow down/up my frequency… maybe i can order a ready oscillator for my purpose?
@jremington: if i take a simple crystal oscillator i will have problems with the matching (mixer is SMA 50 Ohm), power of oscillator can tuned or is steable ?(mixer needs +7dBm ) . Maybe the square wave is not a problem. I am confused about this
@leon : are you sure about that?i am searching many days to find any circuit analysis of a diode mixer (single or double balanced) but i cannot find. Do you have any notes to see the currents,the transformers,voltages?
Regards,
George
Mini-Circuits mentions it in one of their app notes.
This type http://www.digikey.com/product-detail/e … -ND/827256 of 30 MHz crystal oscillator is very accurate and reasonably temperature stable, but would need a buffer amplifier for your circuit. That could be a simple emitter follower consisting of just an NPN transistor and an emitter resistor. Impedance matching and output power adjustment can be accomplished with a simple resistive divider.
For example, +7 dBm is about 0.5 V rms across 50 ohms, so the “emitter resistor” in the amplifier could be a 470 ohm/51 ohm resistive divider (roughly 10:1 voltage ratio with standard component values) and would have 46 ohms output impedance. More accurate results will require more precise resistors.
The square wave output will have very strong harmonics at 3x,5x, (90 MHz, 150 MHz, …), which could be a problem for downstream circuits. If it is, you would need a 30 MHz low pass filter between the oscillator and the mixer. As waltr pointed out, the amateur radio forums are a good place to look for advice.
Dear jremington ,
first of all many thanks for your reply it is very understood. So, here is from mini-circuits most common questions:
"Q. I am a digital designer, dealing with pulses rather than sine waves. Is it necessary to
furnish only sine waves to a double-balanced mixer?
A. No. A double-balanced mixer operates as a switching device; pulses are fine, and may
even reduce distortion"
So about squares maybe we will not have problems. The crystal oscillator recommended has +/-100ppm freq stability so is 30.000khz +/- 3000khz if i have made the calculations right. This is a great phase noice as i understand. Maybe a LC circuit will help this?
As Leon said, diode mixers work fine with square waves.
The current challenge is enough drive power (+7dBm) and impedance matching to the mixer.
A resistive attenuation pad (‘T’ or ‘PI’) is desired between the drive amp and the mixer. This improves isolation between the oscillator and mixer.
Mixer output matching is also required plus a buffer amp and filtering to reject the undesired mixer products. This will be important to the resulting signal.
I calculate 30 MHz +/- 3kHz. (which looks like you you meant).The crystal oscillator recommended has +/-100ppm freq stability so is 30.000khz +/- 3000khz if i have made the calculations right.
Fovakis,
What are the specs you need for frequency accuracy and frequency stability?
What is going to be the other input signal and the output signal?
This is my transmitter… i was wrong with the calculations so it is +/- 3KHz…i thing it would be good…maybe it need less…i don’t know what frequency sensitiviy a mixer want…at the datasheet they don’t say nothing about is.
About the attenuators how can i know which model and how many dB attenuation i want ? 
i am a beginner so i have some basic questions…at the datasheet of the mixer it has L-I isolation 40 dB typical…L-R 45dB typical…but i cannot understand how many power go from l.c port to if and rf port…
Here is the mixer’s datasheet http://217.34.103.131/pdfs/ZAD-6+.pdf
Thanks all for your replies](Photo Storage)
The mixer is broad band and not frequency sensitive, data sheet specs 3kHz to 100MHz.
Attenuators are typically “T” or “Pi” resistive (google these) and typically -2dB to -6dB.
Again, look up Ham Radio books for more on these details.
The mixer data sheet specs the signal ‘conversion loss’ between ports.
Ok i will look for more details ata ham radio books !!thankss
Hi again !!! I found this DDS seems to fit with my mixer:http://i1284.photobucket.com/albums/a57 … e33fa8.jpg
Here is my transmitter:http://i1284.photobucket.com/albums/a57 … e33fa8.jpg
Is the DDS good for my device? 8)
remember, Square wave oscillators can be rounded with just a capacitor.
Ive built super cheap ($8.00) ham transmitters and contacted australia and new guinea using not much more than a
74HC240 TTL octal tri-state buffer/driver chip and some resistors and capacitors…Square wave and all!
Wow !!! That’s really cool bro !
