1 to 27Mhz pixie transceiver adaptation  - Page 1
Forum section: electronics / rf-microwave

author: HertZ
author_rank: Contributor
timestamp: December 06, 2019, 09:47:28 am
content: Hi everyone,I want to make a cw transceiver on 27Mhz band. So i want to use the famous Pixie 2 with a 27Mhz oscillator. I've find this schematic for 3.6Mhz.I've to change the oscillator and the output filter. The filter is not a problem to calculate. The problem is I don't know how to make the 27Mhz oscillator. What if I just replace the 3.8 Xtal with 27Mhz ? When I search on internet there is so much different designs for making this oscillator and i'm confused ! I've find one that seems easy but i don't know if it will work at 27Mhz, this is this pierce oscillator :What is the best and easiest way for me to convert these schematic for 27Mhz ?thank you

author: iMo
author_rank: Super Contributor
timestamp: December 06, 2019, 10:49:02 am
content: Dou you have a 27.12MHz crystal?https://en.wikipedia.org/wiki/ISM_band

author: HertZ
author_rank: Contributor
timestamp: December 06, 2019, 09:43:01 pm
content: Yes i've got a 27.12Mhz crystal.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: December 06, 2019, 10:49:20 pm
content: Quote from: HertZ on December 06, 2019, 09:43:01 pmIf you use the original oscillator the most critical part is the capacitive divider C1 and C2, if it does not oscillate play with the values of those caps. If i recall right, they set the phase of the feedback loop to get things going. 100puff seems low for 80m, so it might be fine at 11m as is.

author: HertZ
author_rank: Contributor
timestamp: December 10, 2019, 01:44:05 pm
content: Tanks for your replies , i've tested by juste replacing the xtal in the schematic by a 27mhz one. The result is there is great oscillations but it's at 9mhz... it's the fundamental of the quartz i think, even it's a 27mhz in reality it's a 9mhz used at it third overtone. I've tried on a breadboard , maybe this can cause the crystal to not oscillate at the third overtone ? Or do i have to change the schematic before making a pcb prototype ?

author: iMo
author_rank: Super Contributor
timestamp: December 10, 2019, 04:03:28 pm
content: All crystals >25MHz are usually 3rd (5/7th) overtone.You have to force the crystal to 3rd overtone, ie. with a serial LC tuned to 27.12MHz in series with the crystal. Or use a different type of oscillator better suited for overtone operations.PS: you may also put a parallel LC tank in the collector of the Q2 tuned to 27.12MHz. Couple the Q1 via a few turns (ie. 10:1 transformer) at the tank's cold side.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: December 12, 2019, 11:19:07 am
content: Kudos for trying but I don't think you'll get as far as building something from scratch based on an existing 27 MHz design (eg based on an old walkie talkie with superhet rx plus BFO in the receiver).Others have mentioned the issue with overtone crystals. Output power at 27 MHz will be much less than lower frequencies as gain of final transistors drops.The receiver will likely be very deaf if you get any results at all. 27 MHz requires much more gain than a receiver at 7 MHz (which has more noise). Personally I wouldn't even attempt it.Very simple receivers for 27 MHz are superregen. Good for AM but not CW. Unless you modulated the CW with an audio tone in which case it would work OK.

author: HertZ
author_rank: Contributor
timestamp: December 13, 2019, 04:23:30 pm
content: Thanks for your answers i'll see what i can try

author: German_EE
author_rank: Super Contributor
timestamp: December 14, 2019, 07:22:30 pm
content: For 27 MHz operation C1 and C2 are way too big, try about 12pF and see if the crystal then oscillates at the third overtone. If you still have no luck then fit a 27 MHz parallel tuned circuit in the Q2 collector connection.I just spent a happy ten minutes with some online calculators. A 0.3 inch (8mm) diameter coil 0.4 inches (10mm) long with 5 turns will be about 0.105 uH. Wire a 330 pF capacitor across this and it will resonate at about 27 MHz.https://daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtmlhttp://www.1728.org/resfreq.htm

author: iMo
author_rank: Super Contributor
timestamp: December 16, 2019, 03:15:08 pm
content: Quote from: German_EE on December 14, 2019, 07:22:30 pmThe 0.1uH/330pF ratio is rather weird22-33pF and an appropriate L is the typical ratio, afaik..For example 27pF and 1.28uH.PS: the 27MHz band was pretty popular in 50ties-70ties for radio control of models. The transmitter was <1W and the receiver was a super-regenarative stage (amplification ~1million) followed by 3-transistors amplifier, and then by some 1-3kHz LC band-passes (the channels) with servo drivers. It worked 1km max afaik. So do not expect more than 100 meters with "pixie" at this band (or perhaps a ~1km with yagi antennas)