EMRFD Message Archive 8369

Message	Date	From	Subject
8369	2013-03-19 21:51:55	lmeeny	7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22
Hello, I'm confused on how the amplifier has any net gain. Can I make the following assumptions? 1. The antenna source impedance is 50 Ohms 2. The input impedance seen at mixer TUF-3 RF input is 50 Ohms. if so then ... The grounded gate configuration exhibits no current gain, currents through the source and drain are equal for all intents and purposes so the same current from the antenna flows to the mixer. Since both in and and see impedances of 50 Ohms why isn't the gain approximately 1? I'm missing something. 73 Ed W2GHD
8377	2013-03-20 03:40:57	kb1ckt	Re: 7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22
Been a while, but iirc a mixer is not 50 ohm as much as it whatever impedance is presented at the other port. At low frequencies (where the wires inside the mixer are short relative to wavelength) the mixer is being driven to hard on pretty much all the time--for one half of the LO signal the mixer output is in phase with the input signal, and the other half of the period the output is 180 degrees phase shifted. The diodes (or FET's) are driven to be hard on, and ideally are low impedance. A common gate amplifier can be used as an impedance transformer, or as a 1:1 transformer. In this case (don't have the book in front of me) i believe it is being as an isolation amplifier, so as to prevent the antenna from radiating any reflected energy from the mixer--there should be very little gain going the opposite way through the amplifier. Due to the switching noise of the mixer (think about the transition times of the LO signal reversing phase) lots of spurious signals can be made. Shawn kb1ckt Sent from my iPad
8380	2013-03-20 09:19:48	John Marshall	Re: 7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22
Ed, Your assumptions are OK but you are ignoring the impedance transformations provided by the input and output filters. I simulated them using SimSmith and came up with about 200 ohms at the FET source and 600 ohms at the drain. Since the signal current in the drain is the same as in the source, the 3:1 impedance step-up will give a voltage gain of 3 and a power gain of 9. Not exactly the 10 dB shown in the block diagram, but "in the ballpark". BTW, I used SimSmith not because it's better than the tools provided with EMRFD but because it runs seamlessly
8381	2013-03-20 11:36:16	PY8ELO - Silva	Mini Circuits XFRM TC1-1-13MG2+
Hi all, I am looking for the Mini Circuits XFRM TC1-1-13MG2+ and I has found it on the manufacturer web site for a cheap price but the shipment to Brazil is very expensive. I was thinking about purchase 10 pcs and theminum order is 20pcs. Can somebody send me few pcs for a reasonable value please? Thanks in advance and Best 73's, Silva. -- **************************************** Brazilian Ham Radio - ARRL Member Callsign: PY8ELO Name: Silva Grid Locator: GI25ar Address: POBOX 151 CEP 68180030 ITAITUBA - PA - BRAZIL **************************************** "The quieter You become the more You are able to hear"
8383	2013-03-21 20:10:54	Todd F. Carney / ...	Re: 7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22
8384	2013-03-21 20:10:55	lmeeny	Re: 7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22
John, Thanks for the detailed reply. I've been using LTSpice and now that I know my simulation's lied to me perhaps I'll learn something new. Again, thank you, Ed W2GHD