Phase Noise Problem

A number of people have been playing with these over the last few months. One of which is Andy VK2AES who has reported that the connifer has excessive phase noise contributed to by the 7812 three pin regulator. A close look reveals small smd ceramic decoupling capacitors either side of the regulator and a 22uf electro on the output. The National Semiconductor data sheet specifes that if the regulator is a long distance from the main filter capacitor (20Meters of coax definitely qualifies), then a 1uf Tantalum should be placed between ground and the input to the regulator. The cap should be as close as possible to the regulator. Andy has done this mod and reports a dramatic improvement.

I now have a sweeper that covers .1 to 12Ghz  it's time to check the usable frequency of these as well as the Pacific Monolithics downconverters. If need be the removal of the Ceramic filter block's will be lookeed into also.

Some measurements have been made and can be seen here.
 
 
 

This is the Preamp/Downconverter with an internal dipole that mounts in the middle of the reflector a picture of it assembled and on a dual axis rotor can be seen here.
 
 
 
 
 
 
 
 

This is what's inside. Two SMD PCB's back to back in a shielded case. at the end is a center fed dipole made with flat copper strips.
 
 
 
 

This antenna was used to recieve pay TV from black mountain tower in the 2.4Ghz band my rough measurements show 2401Mhz being down converted to 450Mhz. The internal oscillator crystal is labeled 7.6210Mhz . Here's where the guessing starts untill further accurate measurements can be made. 2401Mhz in minus the downconverted frequency leaves us with 1951Mhz when this is divided by 256 'ie: an 8 bit counter in a PLL loop' it gives us 7.62109Mhz. That's damn close to the number on the crystal.A bit of probing shows the crystal running at 7.6212Mhz. A nearby IC has 22.864mhz and 15.243mhz . 1951Mhz divided by 128 'ie:7 bit counter equals 15.24218Mhz Hmmm! that sounds close to what I measured on that chip. half that comes to 7.6215Mhz.

Looks like we can start fiddling with that crystal now and see if we can move the down convert frequency into the ham band !!

The antenna takes DC power from the coax, Positive to the center drawing around 300ma. They have a 12 volt 3 pin regulator (7812) in them which requires a minimum of 2 volts differential in order to regulate. A surplus of between 3 and 6 volts would be fine. 18Volts positive on the center of the coax provided by  a masthead power feed box is what's needed in order to get around any cable losses in long run's. Peter Ellis 'VK1KEP' has drafted up one that I have made. I't details are here. For a look at it heres an image.

This unit is designed for 75 ohm coax so you may want to make a 1/4 wave coaxial transformer at the reciever end to bring it's impedance down to 50 ohm, If you feel the need they are a little tricky to make and not that essential for recieving only.

These unit's are more commonly 18db gain however some have a larger dish of identical construction wich have a gain of 27db. The photo on my Dual Axis Rotator page shows an 18db version. There should be nothing to stop us putting the downconverter with any reflector we like. Mabey later!

1.    To alter the crystal ferquency in order to  bring it's output down into the 70cm band. This seems like a good idea but it could be a problem if you want to listen to your transmissions through the satellite. That is if the up link is 70cm in the case of AO40. I will be sticking to the un modified antenna as I have a Win-Radio and a Yaesu FRG9600 which both are controlled by the computer and work happily at 450Mhz. Andy VK2AES has ordered some crystals and should have one operating In-Band soon!

2.    Placing the antenna in a more complete dish to improve gain. This definitely sounds like a good thing to try. The larger barbecue grill (27Db) is perfect but they are not as common.

3.    A multi gang approach may work. It also provides an easy way to get the impedance down to 50ohms. ie:using a simple phasing harness as described in the ARRL handbook etc.. The big question is- Are they all simmilar enough in their gain/phase properties and down convert frequency to make this possible? I don't think so. It may be possible to feed the 7.6215 crystal from one to drive the others. This sounds like trouble to me, I think I'll try the large dish approach first.

4. Constructing a Helical antenna to feed into the down-converter seems straightfoward enough. Here's my attempt at a helical antenna using the connifer

  Credit goes to Peter Ellis 'VK1KEP' for coming up with the idea of using the connifer antenna for sat work. Thanks Peter. Also Thanks to Andy VK2AES and Carl VK2TP, These guys have real test equipment and are cool with the black art of UHF electronics.
 
 

This Information is provided by Kerry Richens VK1TKR in the pursuit of Amaetur Radio and the free exchange of knowledge and ideas.