EMRFD Message Archive 14069
Message Date From Subject 14069 2017-06-26 19:27:14 rcbuckiii Question grounded gate amplifier for receiver A couple of years ago I built a direct conversion receiver for my daughter. She doesn't have a license but wanted to occasionally listen. The receiver I built was a fairly simple design that covered 40 and 20 meters. I used an L/C VFO and crystal oscillators for the hetrodyne mixers. I used the grounded gate amplifier shown on page 9.35 of EMRFD. I built a simple frequency counter into the radio and used a PIC24 processor to send the readout to an LCD. It performed well and the only downside was it didn't have single signal capability.
Now I am going to build her another receiver but will use a single conversion superhet design. For the crystal filters I will use the Inrad 9 MHz, 4-crystal kits. I am going to use the W7ZOI hybrid cascode amplifier circuit since I have a spare one on-hand. This receiver will cover 40 and 20 and I am also adding 10 MHz WWV as a fixed output.
The question I have is related to the grounded gate amplifier. I used one for both 40 and 20 in the DC receiver. This was mainly to isolate the DC oscillator signal from the antenna. I know I don't need the RF amplifier for 40 because the antenna noise floor will be greater than the receiver noise floor. However, should I include the amplifier for 20 meters since the antenna noise floor there is quieter?
One other question. This is not meant to be a high performance receiver. I was planning on using an AD9851 for the tuneable LO and a VXO for the USB/LSB selection. I will clock the AD9851 with an external 100 MHz oscillator rather than use the internal multiplier. Will the spurs be reasonably well behaved? Or should I use and Si570 and put up with the pops when changing frequency greater than +/- .35%. I could also run the VFO at 2X needed frequency and use a divider chip to divide it back down if that would help with spurs. I will be using a PIC24 to control the VFO rather than an Arduino like most people use.
14071 2017-06-26 22:43:29 Scott Nolde Re: Question grounded gate amplifier for receiver Hi Ray,The RF pre amp depends on the mixer used, is it an active mixer with gain? Or a passive with loss?One of the best designs I have used is the one for the Progressive Receiver, SBL-1 mixer with a strong post mixer amp. Preamp is grounded gate at about 10 db of gain.I will also suggest an RF attenuator at the input to the receiver.If an active mixer is used with gain then probably the preamp is not needed.I would still put selectable attenuation at the receiver input.In my experience the 9850/9851 DDS works well as the LO in a receiver as long as the desired frequency output is one third or less of the reference oscillator. So for 100mhz reference about 30mhz output. Be sure to have the correct LO signal injection power to the mixer for best performance.Just my 2 cents worth, good luck with the receiver.Let us know how it's going.Scottkg4rll 14072 2017-06-26 23:39:05 rcbuckiii Re: Question grounded gate amplifier for receiver Scott, thanks for the comments.
I will be using a Mini-Circuits ADE-1+ for the mixer stage. The output frequencies of the AD9851 will be in the 16 MHz and 23 MHz range since the IF amplifier will be at 9 MHz. So I will be at less than 1/3 of the 100 MHz reference.
I will boost the LO to about 12 dBm and connect the output to separate DTC bandpass filters for 16 and 23 MHz for 40 and 20. The output of the bandpass filters will connect to a 3 dB loss PI pad to provide a 50 ohm match to the ADE-1+.
14075 2017-06-27 12:08:27 Dana Myers Superhet LO (was Re: [emrfd] Question grounded gate amplifier for re 14076 2017-06-27 12:45:03 aa0zz Re: Superhet LO (was Re: [emrfd] Question grounded gate amplifier fo Re: Si5351 vs Si570>Programmed similarly to the Si570, it doesn't have the requirement
>to reset the PLL on frequency changes so may not pop.I don't think this is correct. I couldn't find the number in the Si5351 spec so I'm not sure how far the frequency can change without requiring a reset but it's still required at some point. I have limited experience with the Si5351 but I have programmed it (in PIC assembler) to work. I haven't experimented to see how far the PLL can be pushed without a reset before it gets unstable. Programming is in a "different universe" from the Si570 although much of it can be hidden with Arduino C or using someone else's library.73,-Craig, AA0ZZ
14077 2017-06-27 16:03:52 Dana Myers Re: Superhet LO (was Re: [emrfd] Question grounded gate amplifier fo 14078 2017-06-27 17:31:20 rcbuckiii Re: Question grounded gate amplifier for receiver Dana, I thought about the Si5351A but wondered about the higher phase noise. Maybe it would not be an issue since it isn't a high performance receiver. The 3 outputs looked appealing since the extra 2 outputs could be used for USB/LSB BFO selection. And as you say, the spur issue goes away. I know Pete, N6QW, uses the Si5351 for most of his projects so it must not be that bad.
Craig, programming will not be a problem. I have been programming PICs for the last 22 years. I started out using assembler but switched to C about 10 years ago. I still occasionally program in assembler if I am using one of the smaller parts such as a PIC10F200. I looked at Arduino a few years back but the environment hides too much for me to feel comfortable with it. I looked through a couple Si5351 data sheets and couldn't find any mention of the PLL needing to be reset after frequency changes. However, I believe like you that it is required at some point.
14079 2017-06-27 17:38:40 aa0zz Re: Superhet LO (was Re: [emrfd] Question grounded gate amplifier fo Thanks for diving into this and giving a clear explanation, Dana. As everyone can see, it's messy.The Si570 only requires a Reset when the frequency has changed more than +/- 0.35% from the last Reset. The Reset causes a 10ms dropout in the RF so that's why the pop is heard. On 40M, 0.35% is about 24 kHz so you can move that far up and down very quickly with no resets/dropouts required. It takes a bit of math on every frequency update to determine if you need to do the Reset so sometimes programmers take the easy way out and just do it every time. Not recommended.I know this has also been discussed for some Si5351 applications and the popping sounds from Resets are sometimes very annoying there too.73,-Craig, AA0ZZ 14080 2017-06-27 19:01:31 Dana Myers (Microcontrollers, too) Re: [emrfd] Re: Question grounded gate ampli 14081 2017-06-27 19:08:01 Dana Myers Re: Superhet LO (was Re: [emrfd] Question grounded gate amplifier fo 14082 2017-06-27 19:28:36 Scott Nolde Re: Superhet LO (was Re: [emrfd] Question grounded gate amplifier fo Hi Ray,That scheme for the front end sounds fine, should work well. Down conversion is the way to go. This is how I am doing my version of the progressive receiver.I used the cascode IF amp at 9 MHz with a 2.1 kHz wide xtal filter for ssb and a 500 Hz CW filter.Impedance match the post mixer amp to the filter for proper termination and the input to the IF amp.Do shield the BFO and DDS LO to guard against birdies.What audio amp are you going to use?
14083 2017-06-27 20:28:10 rcbuckiii Re: Question grounded gate amplifier for receiver "ST Micro STM32 family"
Yes, very nice family. I've use STM32F407 for a couple of Ethernet applications. CubeMX and System Workbench made it fairly easy even though it does insulate you from the "bare metal". Some learning curve involved. ARM is overkill for amateur projects unless you are designing a FTDX5000 type radio. Even the PIC32 is overkill in most cases.
"A rate-sensitive VFO makes a lot of sense here"
Something to think about and fairly easy to implement.
14084 2017-06-27 20:41:30 Dana Myers Re: Question grounded gate amplifier for receiver 14085 2017-06-27 20:55:59 rcbuckiii Re: Superhet LO (was Re: [emrfd] Question grounded gate amplifier fo Scott,
Each stage will be in separate shielded boxes connected together with SMA connectors and RG-174. Even the LCD display will be shielded. I had some trouble with noise from the LCD on the DC receiver I built. I finally got rid of the interference by shielding the display.
I will use either a TDA2003 or a LM380 for the audio amp. Both are low distortion parts. The TDA2003 is obsolete but I have about a dozen of them on hand. The only thing the LM386 is good for is target practice.
14091 2017-06-28 09:01:34 Raj Re: Superhet LO (was Re: [emrfd] Question grounded gate amplifier fo I am glad someone agrees with me. I normally use beer tins but I may have enough LM386.
I now prefer TDA2822 the 8pin version bridged mono and yuou get 2W out.
Many cheap ready Mono boards on ebay with and without volume control. I always keep a few handy.
At 28-06-2017, you wrote:
>I will use either a TDA2003 or a LM380 for the audio amp. Both are low distortion parts. The TDA2003 is obsolete but I have about a dozen of them on hand. The only thing the LM386 is good for is target practice.
14092 2017-06-28 09:02:31 Scott Nolde Re: Superhet LO (was Re: [emrfd] Question grounded gate amplifier fo RayAgreed that the 386 is target practice.Good luck with the rig and keep us posted on the progress.
14103 2017-06-29 04:01:59 i7swx Re: Superhet LO (was Re: [emrfd] Question grounded gate amplifier fo Hi Scott,
I am also using the TDA2003. Although and old IC it has a positive option, the possibility to reduce the audio bandwith, something that also reduces audio noise. I have applied mods to the Yaesu FT920 audio amplifier cutting high frequency around 3kHz (no FM option used).
I will add the circuit diagram to files.