Month: July 2014

Simplified switching mixer DC receiver uses no tuned circuits

raycharlesring RF

This receiver is a simplified version of my quadrature sampling receiver. It  is simplified because it does not require the phasing filter section and also does not divide the LO by 4. This simplifies the local oscillator requirements significantly. The receiver uses a 2 pole low pass filter for selectivity and a TDA7052 audio amplifier. The CS2000 is a SPI controlled clock generator and is used for frequency generation and tuning, but any VFO, VXO  or any other stable frequency source will work. In this case, one of the extra inverter stages could be used as a linear amplifier to boost up the oscillator output if required(need a good squarewave). If this is done, the micro-controller can be eliminated. The input RF amplifier can also be eliminated if you are using an approximately 50 ohm resonant antenna. In this case the antenna would connect through C13 to pin 9 of IC5. If you do this – pin 9 must also be biased to 1/2 supply. This can be done with a couple of  4.7k  resistors connected in series from V+ to Gnd. The center  junction point will go to pin 9.

This receiver can receive AM but must be at zero beat(exactly tuned). The CS2000 has a resolution of 1Hz or so and is very stable – so the receiver work pretty well for AM. Unlike the phasing receiver this circuit cannot eliminate one of the side bands, but to be honest its not worth effort unless you want contest grade ham receiver performance.

In the video you can see I break out the power switch, audio gain, tuning encoder and display to a daughter board.

I designed the display to have very low spurious noise. The info on this is here:

https://circuitsalad.com/2014/07/31/very-low-power-3-digit-lcd-display-with-serial-control/

More info about how this circuit works can be found in this post, which is the quadrature version of this receiver.

https://circuitsalad.com/2013/12/30/my-phasing-receiver-is-a-success/

Schematic:

DC Receiver

Demo Video:

zip file of code: Filedropper is full of dumb ads but look for the “Download This File” button in middle of screen

http://www.filedropper.com/rcvrfirmware2

Very Low power 3 digit LCD Display with serial control

raycharlesring micro controllers

I had a need for a LCD display that consumed very little power and generated very little digital noise . You can buy LCD’s with driver chips but I have had great difficulty with these modules because they create excessive wide band noise.

To solve this problem, I made an LCD driver out of a 18F26K22 PIC micro-controller and a low cost static LCD display module. A static module requires more pins( a pin for every segment) but the drive logic is easier to implement. You cannot simply drive the the LCD segment high or low, as with a seven segment LED display. Once the capacitance of the display charges up, the segment contrast fades. The solution is to cycle a given segment on and off  at rate fast enough not to strobe(100Hz in my case).

In my design, the  display is driven by a four byte serial data packet which consists of: <startbyte>,<digit1>,<digit2>,<digit3> . The data bytes are sent as rs232 data at 9600 baud.

The startbyte is one of  three values(shown here in HEX): oxA0(no decimal point), 0xA1(decimal point 1st digit), 0xA2(decimal point 2nd digit). The valid data bytes are 0-9(non ASCII) and ASCII characters, “A”, “C”, “E”,”F”, “H”, and “P”. If you send any other values for a given digit, it will clear(blank the digit)

The module is clocked using the internal RC clock at 1Mhz and only draws 500uA when operating. It must be operated at greater than 4v or the LCD will be faint. If using a 3v supply, you can use a simple Dickson voltage doubler circuit to generate the supply voltage.

Picture of module(LCD covers the micro-controller):

LCDisplay

Demo Video: (Radio Receiver using the display module)

Schematic Diagram:

LCD_MOD_3Char

Link:   Zip File with ExpressPCB layout file and HEX file

http://bit.ly/1rDKMx1