Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Feb Sat 22, 2020 8:19 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 31 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Fri 03, 2020 9:15 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 236
The thing about a Franklin oscillator is that by using TWO active devices, we can get a 360 Degree phase shift without ticklers or tapping coils(Hartley) or tapping caps(colpitts and variations including seiler, clapp, vackar) and are not dependent on the phase shift of the tuned circuit to provide part of the required phase shift.
Only to Set a frequency.

This means the Franklin can oscillate at darn near ANY frequency the active devices are capable of amplifying.

I am eager to see how well the 2n3904 performs.


Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Fri 03, 2020 9:31 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 236
Frequency drift of my FM TX in a 24 hour period when emitter resistor was increased to 100ohm in T4 final stage. Po = 5mW

10:19am
89.2212 mhz

11:04am
89.2206 mhz

Room heater on

11:46am
89.2222 mhz

12:00pm
89.2226 mhz

1:10pm
89.2232 mhz
Heater off

1:18pm
89.2229 mhz

1:24pm
89.2230 mhz

2:39pm
89.2232 mhz

2:52pm
89.2231 mhz

3:09pm
89.2229 mhz

3:50pm
89.2222 mhz

4:15pm
89.2228 mhz

4:34pm
89.2224 mhz

5:04pm
89.2222 mhz

5:08pm
89.2221 MHz

6:25pm
89.2222 mhz

6:38pm
89.2216 mhz

7:47pm
89.2213 mhz

7:54pm
89.2207 mhz

8:16pm
89.2216 mhz

8:28pm
89.2218 mhz

10:07pm
89.2208 mhz

11:04pm
89.2203 mhz

11:11 heater switched on

11:26pm
89.2216 mhz

11:27pm.heater off

12:44am
89.2208 mhz

8:26am
89.2216 mhz

9:42am
89.2218 mhz

10:22am
89.2219 mhz

11:02am
89.2217 mhz


Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Fri 03, 2020 2:20 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 236
https://ibb.co/Qc2NZbR

https://ibb.co/swfPpGh (Schematic updated)

Final amplifier prototype and schematic.

Driver transistor is operating in common base configuration for maximum input output isolation. Output power without a low pass filter is 550mW at 89.1MHzbut it should drop to 400mW with a good low pass filter. Not bad for such a simple design.

This amplifier will also boost the output of other 5 to 10mW low power FM transmitters.

To tune the amplifier, first complete the driver section and tune trimmer VC1 and spread coil L1 for maximum voltage reading at point V1. Don't connect the final transistor yet. When you get maximum voltage reading at V1 leave it at that and solder transistor T2. Then tune VC2 and spread coil L2 for maximum peak voltage reading at point V2. Remove the 50ohm dummy load consisting of two 100ohm resistors. The amplifier is now ready.

This amplifier is working with my High Stability FM Transmitter. I have tested its output between 88-90MHz into a 50ohm dummy load and it's stable.

If someone can improve on the output matching network of L2 and VC2 for more power and efficiency then let me know. The final transistor is quite hot.

Update: Feeding the output into a 75ohm dummy load is causing significantly lees heating of the PA transistor T2. A simple half wave dipole is easy to construct and its input impedance is 75ohm. Once I used a PVC pipe 160cm long and I taped two wires each measuring 78cm. The center was fed with a coaxial cable from my 96MHz PLL FM TX. The antenna performed well and I was able to pick up the signal more than 1 mile away while the antenna was placed in my living room. Mounting it 20 feet above ground would have tripled the range. At VHF frequencies antenna height is everything.


Last edited by Dare4444 on Jan Sat 04, 2020 10:08 pm, edited 4 times in total.

Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Fri 03, 2020 11:38 pm 
Member
User avatar

Joined: Jun Sun 22, 2008 2:03 am
Posts: 1451
Location: Burke, VA 22015
Nice work! From your data, the drift is less than 3 kHz or 33 ppm which is quite impressive. Maybe by using a better transistor for T1 such as the 2SC9018, SS9018 or J211, the frequency stability could be improved further. BTW, it looks like the oscillator you have is a Seiler circuit rather than a Colpitts.

_________________
Binh


Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Sat 04, 2020 4:25 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 236
bb.odin wrote:
Nice work! From your data, the drift is less than 3 kHz or 33 ppm which is quite impressive. Maybe by using a better transistor for T1 such as the 2SC9018, SS9018 or J211, the frequency stability could be improved further. BTW, it looks like the oscillator you have is a Seiler circuit rather than a Colpitts.


Thank you for appreciating my work. The circuit is now ready for publication. I did a Google search and indeed it's a Seiler oscillator.


Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Sat 04, 2020 1:11 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 236
Macrohenry wrote:
Good work! Might I suggest your next contribution is to build a stable MW signal source that can be substituted for the crystal in the 386 and 6888 transmitters. Maybe add a 1:100 divider to your FM oscillator to get less than 70 HZ drift on MW.


Two 74AC74 and one CD4040 chip will divide the 88-89MHz by 128 for coverage from 687.5 KHz to 695.3KHz.

The chips can also divide by 64 by selecting /8 output of CD4040. Frequency coverage would be 1375KHz to 1390KHz

Maximum drift would be around 40 and 80Hz respectively for 5KHz total drift of FM oscillator.

Direct frequency generation using Franklin topology would be a better choice though. If carefully built then drift would be less than 50Hz


Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Sat 04, 2020 2:01 pm 
Member
User avatar

Joined: Sep Fri 14, 2018 7:56 pm
Posts: 37
Location: Switzerland
The first coil for L1 wasn’t so stable as I meant in the first time. I tried different ones, one with 3 mm/0.12’’ diameter 11 turns SWG26 is now stable at constant room temperature. After soldering the coil carefully without mechanical tension, a temperature coefficient of 2.5kHz/K was the result. That is maybe the same as you observed.

Two sources I read say temperature coefficient is negligible for air coils.
One source from 1969 mentions average currents from 2 to 5 mA for transistor vhf oscillators.
I will now investigate other parts like the crappy trimpot and condensers and try bigger values for R9/R10.

Btw. I wouldn’t recommend superglue (cyanacrylate) for fixing coils because it lowers their Q-factor by about 10%. Better use beeswax or adhesives on the base of polystyrene or polychloroprene (contact glue). Glues containing water resp. beeing hygroscopic halfen the Q.
For this reason I’d like to avoid the paper ear bud sticks.

Regards Marc


Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Sat 04, 2020 7:18 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 236
Hi Marc,

Are ceramic formers better for winding coils?
Copper has a higher temperature coefficient than iron or silver. I did experiment with 3T of 19AWG magnet wire on 8mm air core and heated it gently with a matchstick and its inductance went up 1nH. It means a change of 100KHz 96MHz!

In my circuit when I removed the capacitor between base and emitter then stability improved dramatically but without the capacitor the audio did sound a little distorted. If you warm that capacitor with your finger you would notice the frequency going up. The capacitors coupling the base to coil and coil to zener are also very temperature sensitive. NP0 capacitors should help and so will a coil etched on the PCB.

The 16V zener can be sensitive too to temperatures. My Friend used three 5.6V zeners in series with 470K resistors between them to distribute the current equally and it was much better than a 16V zener. The diode was causing drift in his FM TX and with three zener diodes it improved dramatically.

In my experiments, when I used a 1K emitter resistor the frequency started drifting. 800ohms was about the right value and I found it by trial and error.

Some hams used to place the VFO in a small air tight styrofoam box and heat the interior to a constant 55C with the help of a temperature controller (Opamp circuit) and a 47Ohm 5W resistor. The VFO would be mega stable and immune to ambient temperature changes.


Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Tue 21, 2020 1:35 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 236
This is the FM oscillator in my circuit.

https://ibb.co/987cZCp

When I used all NP0 capacitors then stability was poor. Then I used NP0 for only C1 and C2 while C3 and C4 are normal ceramic disks and Stability is excellent now. The capacitance of C4 changed a lot when I heated it with a soldering iron. C4 was connected to a LCR meter for testing. So the frequency in a VFO depends a lot on the combination of capacitors used.

The coil is very tightly secured with superglue. Touching the coil and warming it with my fingers only shifts the frequency upwards by 3-4KHz or so.

So all NP0 capacitors might not be a good idea for a VFO. The right combination has to be chosen by trial and error for maximum stability.

It's an art.

Replacing C4 with high quality parallel NP0 capacitors caused the oscillator to drift while using cheap ceramic capacitor for C4 is giving good stability. Weird and counterintuitive.


Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Wed 22, 2020 9:18 pm 
Member
User avatar

Joined: Sep Fri 14, 2018 7:56 pm
Posts: 37
Location: Switzerland
Hi
I apologize for not having answered, have to work again.
I came to the same conclusion but I'm still testing ... yes it's an art.


Regards
Marc


Top
 Profile  
 
 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Wed 22, 2020 10:53 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 236
Popov wrote:
Hi
I apologize for not having answered, have to work again.
I came to the same conclusion but I'm still testing ... yes it's an art.


Regards
Marc


Hi Marc,

Please replicate the original coil. 14T 25SWG tightly wound on a piece of 2mm diameter Johnson ear bud. Secure with superglue. Increase emitter resistor of T4 to 100ohm for 4mW output. Must use 2n3904 for oscillator transistor. The base emitter junction capacitance changes with temperature and the capacitor values help compensate for it and that's the reason for its stability. Putting the circuit in an airtight enclosure will improve it even further.

Capacitors

across emitter resistor = 15pf NP0
Base - emitter = 22pf NP0
Base to coil = 10pf ceramic from junk box
Coil to zener = 40pf - 56pf cheap quality ceramic from junk box.

I just tested it again with the above values. The circuit board is not covered. Here's the frequency reading.


1:03am
92.5918 mhz

1:14am
92.5909 mhz

1:22am
92.5920 mhz

1:23am
92.5917 mhz

2:40am
92.5924 mhz

2:44am
92.5920 mhz

3:03am
92.5915 mhz

3:07am
92.5912 mhz

3:13am
92.5905 mhz

3:50am
92.5921 mhz

4:56am
92.5927 mhz

5:11am
92.5903 mhz

5:23am
92.5898 mhz

9:07am
92.5925 mhz

12:59pm
92.5925 mhz


Drift is 3KHz in twelve hours.

In a previous test I got a drift of 5KHz in a 24 hour period. The board was open and not shielded against air currents.

This is as good as it gets without using a JFET. This TX can be made into a nice little kit with PCB inductor for the oscillator coil.

I built an VHF amplifier just for the sake of it. Here's the schematic.

5mW input for 0.6W output without a low pass filter.

https://ibb.co/1dKBGLs

New updated final schematic. It has been tweaked for stability.

Output is 600mW at 92MHz into a 72ohm dummy load. Both transistors are biased for class AB operation for a cleaner output signal. Driver stage is a common base amplifier for maximum input output isolation.

Gain is around 20dB.

Here's the FM TX driving the amplifier.

https://ibb.co/Js5Zt8N

That's it. I am done with the project.


Top
 Profile  
 
Post New Topic Post Reply  [ 31 posts ]  Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 3 guests



Search for:
Jump to:  






























-->


Privacy Policy :: Powered by phpBB